Adopt latest R.Net for concurrency fixes

The latest R.Net 1.5.12 and RDotNet.FSharp 0.1.5 include concurrency
fixes that resolve issue #64. They might also resolve RInteropServer
disconnection issues that have been seen, though this will take some
further testing to verify.
Use nuget packages for R.NET 1.5.12
Merge branch 'feature/new-RDotNet' of https://github.com/dcharbon/FSharpRProvider into rdotnet_1.5.11

Conflicts:
	nuget/RProvider.nuspec
Remove superseded R.NET references