title: Can't find UnityEngine.UI namespace after installing Visual StudioTools for Unity
body: <p>I have been using Visual Studio 2015 community (version 14.0.25123.00 Update 2) with .Net framework v4.6.01038 with Unity Tools for VS2015 which I used to debug my unity projects. Everything seemed to work fine but after I tried to work with UnityEngine.UI VS start showing errors saying that the namespace can't be found. I have used the exact same project files in to another pc where the only difference was in .Net framework version (v4.6.01055)and that worked. If I remove the unity tools from my previous pc, the build error goes away but I guess there is no way to debug without the unity tools. Is this a issue with .net framework or I am missing something? 
   - I tried to export and re-import all unity assets and packages but that did not solve the issue
   - VS2015 was uninstalled and reinstalled from scratch - still did not work
   - unity tools was also reinstalled
   - Also deleted the projectFolder\Library\UnityAssemblies folder and restarted VS2005.</p>
  
  <p>None of these methods seems to work for me. Tried to find the exact version of  .net framework but failed too. Is there a specific solution to this problem?</p>