Change AssemblyVersion to 2.7.5.0

From [CodePlex 35119](https://ironpython.codeplex.com/workitem/35119), this changes the `AssemblyVersion` to 2.7.5.0 from 2.7.0.40. In addition, it includes publisher policy files for IronPython.dll and IronPython.Modules.dll that are installed into the GAC alongside the actual assemblies.

This has a few consequences:
* Running old versions of ipy.exe will use the assemblies in the GAC. This has always been the case, but since the version numbers never changed it wasn't obvious.
* Anything built against 2.7.0.40 will now load 2.7.5.0 (thanks to the publisher policy). Again, this is how it would be without changing the AssemblyVersion, but it's now more obvious.
* Anything linked against 2.7.5.0 will no longer try to load older versions. This is the main change, and the reason for making this change.

In all I think this is an improvement but I want to see if anyone sees anything I haven't noticed.