Use rb_wait_for_single_fd instead of deprecated rb_thread_select

rb_thread_select has been removed in ruby 2.2 (see #194 for example). The old code will still be used if `rb_wait_for_single_fd` is not available (which I believe is since 1.9.3)

I've done a quick check that everything still works as intended (linux & os x) when using events, just using the `simple.rb` example in the god documentation.

There doesn't seem to be any test coverage for this though, so extra pairs of eyes etc. appreciated