Use "Preserve Newest" option on OpenTK.dll.config

OpenTK.dll.config was previously set to be always copied to the
output directory. This caused constant rebuilds in Visual Studio.

Setting this to "Preserve Newest" improves build times
significantly.
[Build] Updated mono version to 3.4.0.
Merge pull request #156 from thefiddler/buildfix

Use "Preserve Newest" option on OpenTK.dll.config