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.
Yes. Originally Rbx was *not* in the allow-failures list. I added it later because something wasn't working correctly under Rubinius (I don't remember what). I regularly look at the Travis build output and Rbx has been all green for a while now (with the exception of this one test). Since we seem to have a number of Rubinius users in our community I thought we could elevate Rbx again and see how that plays out.