Bump CustomBuildTasks to .NET 4.0

The bindings are already here, and this also allows us to build both on
mono 3 as well as mono 4, which dropped pre-4.0 assemblies from the
install.

Author: Therzok

Lib/CustomBuildTasks/CustomBuildTasks.csproj

@@ -9,7 +9,7 @@
     <AppDesignerFolder>Properties</AppDesignerFolder>
     <RootNamespace>CustomBuildTasks</RootNamespace>
     <AssemblyName>CustomBuildTasks</AssemblyName>
-    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
     <FileAlignment>512</FileAlignment>
     <TargetFrameworkProfile />
   </PropertyGroup>
@@ -33,7 +33,7 @@
   <ItemGroup>
     <Reference Include="System" />
     <Reference Include="Microsoft.Build.Framework" />
-    <Reference Include="Microsoft.Build.Utilities" />
+    <Reference Include="Microsoft.Build.Utilities.v4.0" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="GenerateUniqueIdentifierTask.cs" />