Updated to .net core 2.0

Author: HeyJoel

build.cake

@@ -47,7 +47,7 @@ Task("Patch-Assembly-Version")
 
     Information("Building version {0} of Cofoundry.", versionInfo.InformationalVersion);
 
-    isPrerelease = !string.IsNullOrEmpty(versionInfo.PreReleaseNumber);
+    isPrerelease = versionInfo.PreReleaseNumber.HasValue;
 
     // Patch the version number so it's picked up when dependent projects are references
     // as nuget dependencies. Can't find a better way to do this.

src/Cofoundry.Plugins.Mail.MailKit/Cofoundry.Plugins.Mail.MailKit.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard1.6</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
 
     <PackageId>Cofoundry.Plugins.Mail.MailKit</PackageId>
     <Description>Cofoundry mail services that utilize the cross platform MailKit library.</Description>
@@ -16,7 +16,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Cofoundry.Core" Version="0.2.0-beta0022" />
+    <PackageReference Include="Cofoundry.Core" Version="0.2.0-beta0054" />
     <PackageReference Include="MailKit" Version="1.16.2" />
   </ItemGroup>