TEST: Fix Add-Type tests on Mono 2.10.8.1 Travis CI.

On Travis we get an error running two tests:

Test Error : ReferenceTests.AddTypeCommandTests.AddTypeAddsAssemblyToCurrentAppDomain
Test Error : ReferenceTests.AddTypeCommandTests.TypesFromAddedAssemblyAvailable

System.IO.FileNotFoundException : Could not load file or assembly
'Microsoft.Build, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
or one of its dependencies. The system cannot find the file specified.
at System.AppDomain.Load

Switch to using System.Web instead of Microsoft.Build in the test.