Update to netcoreapp1.2 once we figure out why referencing the newer project causes running the tool to fail

[pranavkm]

Tracking CoreFx issue: https://github.com/dotnet/corefx/issues/14696.

Also need to re-enable tests once this issue is resolved.

[pranavkm]

Once we have a solution, aspnet/Coherence@438590f needs to be undone.

[pranavkm]

With no changes, (well the transitive dependencies targeting CoreFx 4.4.0-* packages), invoking dotnet test on any of the test projects in this repo is currently failing with:

Unhandled Exception: System.IO.FileLoadException: Could not load file or assembly 'System.Runtime, Version=4.2.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
   at Xunit.Runner.DotNet.Program..ctor()
   at Xunit.Runner.DotNet.Program.Main(String[] args)
SUMMARY: Total: 1 targets, Passed: 0, Failed: 1.

Spoke with @Eilon offline and we decided it was ok to disable tests until we have a resolution for this.

[pranavkm]

Commit to be reverted to re-enable tests: f9efbf8

[pranavkm]

Switching to csproj resolved this issue. Addressed as part of the migration in 3f5cce4