Error: Unable to activate mongo-1.12.0, because bson-2.3.0 conflicts with bson (= 1.12.0) when installing both ruby gem mongo and mongoid

Cause: mongo gem requires bson 1.12.0 while mongoid version > 3.0.0 requires moped which needs bson 2.3.0

Solution: Use mongoid < 2.8.1 if you want to use mongo, or dump mongo and switch to moped:

gem install mongoid -v 2.8.1