Updated configurations node in project.json
- Configurations no longer support target frameworks
- Changed casing of debug/release to Debug and Release by default
Added ability to specify version of CoreCLR to use during build
* Change "configurations" to "frameworks" in test
* Don't pass in compilationOptions to Project.BuildTargetFrameworkNode
  since it's always null at the caller.
Updating configurations to frameworks token in ProjectFacts
Updating release Nuget.config
Updating dev Nuget.config
Updating Roslyn compilation to default to experimental LanguageVersion
Updating tests to reflect Experimental switch