Exclude GC-dependent ThreadLocalVar test on Rbx

Addresses #209 

This test depends on garbage collection being run. The garbage collector in Rubinius runs concurrently with running code. Therefore the `GC.start` call cannot be guaranteed to complete before the expectation is checked. This makes the test fail consistently on Rubinius. The logic *should* work the same on Rubinius since the code is pure Ruby. So this PR guards the test so that it only runs on MRI.