Use VS2015 SDK tasks when StartProgram is VS2015

Conditionally use `VisualStudio.Sdk.BuildTasks.14.0` when
`StartProgram` contains the string `Microsoft Visual Studio 14.0`.

Author: jcansdale

src/GitHub.VisualStudio/GitHub.VisualStudio.csproj

@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="14.0">
   <Import Project="..\..\packages\LibGit2Sharp.NativeBinaries.1.0.164\build\LibGit2Sharp.NativeBinaries.props" Condition="Exists('..\..\packages\LibGit2Sharp.NativeBinaries.1.0.164\build\LibGit2Sharp.NativeBinaries.props')" />
-  <Import Project="..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props" Condition="'$(VisualStudioVersion)' == '14.0' And Exists('..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props')" />
-  <Import Project="..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.props" Condition="'$(VisualStudioVersion)' == '15.0' And Exists('..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.props')" />
+  <Import Project="..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props" Condition="$(StartProgram.Contains('Microsoft Visual Studio 14.0')) And Exists('..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props')" />
+  <Import Project="..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.props" Condition="!($(StartProgram.Contains('Microsoft Visual Studio 14.0'))) And Exists('..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.props')" />
   <PropertyGroup>
     <!-- This is added to prevent forced migrations in Visual Studio 2012 and newer -->
     <MinimumVisualStudioVersion>$(MSBuildToolsVersion)</MinimumVisualStudioVersion>
@@ -27,7 +27,7 @@
     <RootNamespace>GitHub.VisualStudio</RootNamespace>
     <AssemblyName>GitHub.VisualStudio</AssemblyName>
     <StartAction>Program</StartAction>
-    <StartProgram>$(DevEnvDir)\devenv.exe</StartProgram>
+    <StartProgram  Condition=" '$(StartProgram)' == '' ">$(DevEnvDir)\devenv.exe</StartProgram>
     <StartArguments>/rootsuffix Exp</StartArguments>
     <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
     <ZipPackageCompressionLevel>Normal</ZipPackageCompressionLevel>
@@ -800,6 +800,6 @@
     <Error Condition="!Exists('..\..\packages\LibGit2Sharp.NativeBinaries.1.0.164\build\LibGit2Sharp.NativeBinaries.props')" Text="$([System.String]::Format('$(ErrorText)', '..\..\packages\LibGit2Sharp.NativeBinaries.1.0.164\build\LibGit2Sharp.NativeBinaries.props'))" />
   </Target>
   <Import Project="..\..\packages\SQLitePCL.raw_basic.0.7.3.0-vs2012\build\net45\SQLitePCL.raw_basic.targets" Condition="Exists('..\..\packages\SQLitePCL.raw_basic.0.7.3.0-vs2012\build\net45\SQLitePCL.raw_basic.targets')" />
-  <Import Project="..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.targets" Condition="'$(VisualStudioVersion)' == '15.0' And Exists('..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.targets')" />
-  <Import Project="..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets" Condition="'$(VisualStudioVersion)' == '14.0' And Exists('..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets')" />
+  <Import Project="..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.targets" Condition="!($(StartProgram.Contains('Microsoft Visual Studio 14.0'))) And Exists('..\..\packages\Microsoft.VSSDK.BuildTools.15.0.26201\build\Microsoft.VSSDK.BuildTools.targets')" />
+  <Import Project="..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets" Condition="$(StartProgram.Contains('Microsoft Visual Studio 14.0')) And Exists('..\..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.215\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets')" />
 </Project>