Modified to not attempt to run NUnit v3 tests

I also updated the version to 2.6.4 in case we release this. The only real changes are in TestAssemblyBuilder.cs where I check if the test assembly references nunit.framework or nunitlite with a version > 2.

There are large formatting changes because tabs got replaced with spaces.

I did not add unit tests because it would require too much setup and a copy of NUnit v3 tests. I tested by attempting to run both v2 and v3 tests and combinations of the both. All work fine, no tests are found in the v3 assemblies.

The only thing I don't like about this is that it silently drops v3 tests. The only way I know to surface the error is through an exception, but that stops the entire test run.

I am pretty sure that if we drop this in the current Visual Studio adapter, it will start correctly ignoring v3 tests. This might warrant another release of the adapter.