Upgrade trogdir_models to 0.7.1

Upgrades to mongoid 4.0
Upgrading some outdated gems
Removing old Ruby versions from Travis

better_errors was incompatible with 1.9.3 and we don't really need
anything other than what we're running.
Upgrade to rspec 3.1
Merge pull request #2 from biola/upgrade_trogdir_models

Upgrade trogdir_models to 0.7.1