Fix deprecation warning on ruby 2.x

On newer versions it should use rb_thread_call_without_gvl()

    ext/couchbase_ext/multithread_plugin.c: In function ‘loop_run_poll’:
    ext/couchbase_ext/multithread_plugin.c:772:5: warning: ‘rb_thread_blocking_region’ is deprecated (declared at .../2.0.0-p247-dbg/include/ruby-2.0.0/ruby/intern.h:839) [-Wdeprecated-declarations]
         rb_thread_blocking_region(loop_blocking_poll, args, RUBY_UBF_PROCESS, NULL);

Change-Id: I745e6c941378be3741ae426cce2a4fb7cd34dc5e
Reviewed-on: http://review.couchbase.org/28207
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>