RUBY-701 fixing missing pid attribute on unix socket type

One of these things is not like the others. This missing pid attribute on Mongo::UnixSocket was causing Pool#checkout to always issue a new new socket instance when using unix domain sockets.
Merge branch '1.x-stable-ruby-676' of github.com:gjmurakami-10gen/mongo-ruby-driver into gjmurakami-10gen-1.x-stable-ruby-676