use 4.5 framework by default

Author: dsyme

src/fsharp/FSharp.Compiler.Service/FSharp.Compiler.Service.fsproj

@@ -635,11 +635,21 @@
     <Reference Include="System.Numerics" />
     <Reference Include="System.Runtime.Serialization" />
     <Reference Include="System.Xml" />
-    <Reference Include="Microsoft.Build.Framework" />
-    <Reference Include="Microsoft.Build.Engine" />
-    <Reference Include="Microsoft.Build" />
-    <Reference Include="Microsoft.Build.Utilities.v4.0" />
-    <Reference Include="Microsoft.Build.Tasks.v4.0" />
+    <Reference Include="Microsoft.Build.Framework, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="Microsoft.Build.Engine, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="Microsoft.Build, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="Microsoft.Build.Utilities.v12.0">
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="Microsoft.Build.Tasks.v12.0">
+      <Private>True</Private>
+    </Reference>
     <Reference Include="FSharp.Core, Version=$(TargetFSharpCoreVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
       <Private>False</Private>
     </Reference>