References: Resolves warning issue

On building the solution, WebEssentialsTest project throws three warnings:

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Warning 1 A reference was created to embedded interop assembly 'c:\Windows\assembly\GAC\stdole\7.0.3300.0__b03f5f7f11d50a3a\stdole.dll' because of an indirect reference to that assembly created by assembly 'c:\Program Files (x86)\Common Files\Microsoft Shared\MSEnv\PublicAssemblies\envdte.dll'. Consider changing the 'Embed Interop Types' property on either assembly. C:\Users\Adeel\Source\Repos\WebEssentials2013\WebEssentialsTests\CSC WebEssentialsTests
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Warning 2 A reference was created to embedded interop assembly 'c:\Windows\assembly\GAC\stdole\7.0.3300.0__b03f5f7f11d50a3a\stdole.dll' because of an indirect reference to that assembly created by assembly 'c:\Program Files (x86)\Common Files\Microsoft Shared\MSEnv\PublicAssemblies\envdte80.dll'. Consider changing the 'Embed Interop Types' property on either assembly. C:\Users\Adeel\Source\Repos\WebEssentials2013\WebEssentialsTests\CSC WebEssentialsTests
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Warning 3 A reference was created to embedded interop assembly 'c:\Windows\assembly\GAC\stdole\7.0.3300.0__b03f5f7f11d50a3a\stdole.dll' because of an indirect reference to that assembly created by assembly 'c:\Program Files (x86)\Microsoft Visual Studio 12.0\VSSDK\VisualStudioIntegration\Common\Assemblies\v4.0\Microsoft.VisualStudio.Shell.12.0.dll'. Consider changing the 'Embed Interop Types' property on either assembly. C:\Users\Adeel\Source\Repos\WebEssentials2013\WebEssentialsTests\CSC WebEssentialsTests

This PR fixes it.