RUBY-714 fixing missing require for time in bson/date_time

Props to @tylerbrock for catching this one, but the most recent version of bson is missing a needed require for `time` in `bson/date_time`. This is raising an exception for anyone trying to use this gem outside of an environment where that's already in the loaded.

I'd like to get this patched and push a 2.1.1 today to address this. Will do that as soon as this is :+1:  and merged.