upgrade MSpec reference to .NET 4

Author: chester89

src/FluentNHibernate.Specs/FluentNHibernate.Specs.csproj

@@ -57,7 +57,7 @@
   <ItemGroup>
     <Reference Include="Machine.Specifications, Version=0.5.15.0, Culture=neutral, processorArchitecture=MSIL">
       <SpecificVersion>False</SpecificVersion>
-      <HintPath>..\packages\Machine.Specifications.0.5.15\lib\net20\Machine.Specifications.dll</HintPath>
+      <HintPath>..\packages\Machine.Specifications.0.5.15\lib\net40\Machine.Specifications.dll</HintPath>
     </Reference>
     <Reference Include="NHibernate, Version=3.3.1.4000, Culture=neutral, PublicKeyToken=aa95f207798dfdb4, processorArchitecture=MSIL">
       <SpecificVersion>False</SpecificVersion>