Mongo 1.8 renamed send_message_with_safe_check

Found this out here: https://github.com/freerange/mongo-rails-instrumentation/commit/823def1de4d4853ea39a0240750ce8a200d65661.

Not sure if all the changes are necessary, but they seem to work for my app.
Merge pull request #1 from AquaGeek/patch-1

Mongo 1.8 renamed send_message_with_safe_check