Add .ruby-version and .ruby-gemset

Hopefully you guys are amenable to adding .ruby-version and .ruby-gemset files to the inaturalist code base. I use RVM to separate my bundle installs and having these files makes it easier for me to collaborate.

I've set .ruby-version to the latest patch release of 1.9.3, but it's easy enough to change it to another version if you have a different standard.