Appraisals: fix build on Ruby 2.2.0
Appraisals: fix build on Ruby 2.2.0

CircleCI pulls Ruby 2.2.0 but we expect 2.2.2. Because of this the i18n lock to
1.4 has no effect and it makes the build fail:

https://circleci.com/gh/airbrake/airbrake/7325

nick-desteffen/airbrake,airbrake/airbrake,nick-desteffen/airbrake,airbrake/airbrake