Bump hiredis to 0.11.0
Update URL and summary in gemspec
Use rb_fd_thread_select if available

The rb_thread_select function will be deprecated for newer versions of
Ruby (>= 2.1 AFAIK). To preemptively fix this, use rb_fd_thread_select
if it is available.

Also see: https://bugs.ruby-lang.org/issues/9502#change-45212

Thanks to @camilo in #20 for the initial version of this patch.
Use version of Rake that still works with MRI 1.8.7