Customize Mongo and RVM installations

Mongo:
- Allow custom formula to be installed (e.g. referencing a specific SHA)
- Update mongod.conf to disable table scans

RVM:
- Don't hardcode any default Rubies
- Remove unnecessary recipe and bash_it feature
- Simplify installation command to instructions from rvm.io
- Add node-customized gemsets for the default Ruby
Merge pull request #4 from Yesware/customize-mongo-rvm

Customize Mongo and RVM installations