Makes the project compatible with 2012

I just changed all the refs to version "12.0" to "11.0".  Seems like it works fine in 2012 now, but I don't have 2013 to test with to make sure it still works there.

If this isn't a good way to do this, is there a better way to manage extensions across different versions of VS?  A separate project for each version with the files linked in?  Just thinking out loud.

Thanks,
John Bubriski
The MinimumVisualStudioVersion value needs to always take the value of the current version of Visual Studio in order to prevent the upgrade wizard from showing.  The name of the value is very misleading, or at least it's an odd value for the upgrade wizard to use. 

 In it's current form this will likely prevent usage within Visual Studio 2013.  To fix this just use the following pattern instead 

https://github.com/jaredpar/RoundTripVSIX/commit/42da01bc808f315cefd239d97907b4ca6f39d339