PROTON-752: Release the GIL on blocking calls in Ruby.

Added a wrapper call for pn_messenger_recv, pn_messenger_send and
pn_messenger_work that calls the appropriate method for the version
of Ruby to release the GIL until the call returns in C.

Added to both CMakeLists.txt for the Ruby bindings, and also to the
extconf.rb file for the gem, a macro named RUBY## (where ## is 18, 19,
20 or 21 depending on the version of Ruby) to ensure the right API is
compiled into the extension since the API is different between 1.9 and
2.x and non-existent in 1.8, where it's a no-op.