Do not build native extension on OSX

We get these runtime failures on OS X when we use the Thrift native
extension, so rather than invest time in determining why it fails, we
will just avoid building the native extension when installing on OS X.

This will be fine for a long-term solution since the goal will be to
remove Thrift altogether eventually.
Merge pull request #1 from sermo/rg_no_native_on_osx

Do not build native extension on OSX