Allow reporting of custom envs to airbrake

[garettarrowood]

Adhearsion does not properly support custom environments other than the four hard-coded ones (test, development, staging, production). This means that adhearsion-reporter cannot correctly report the devtbX environment names to Airbrake. In order to avoid having to make complex changes to Adhearsion, we'll simply permit an arbitrary override, Reporter.config.environment

Reference: https://github.com/adhearsion/adhearsion/issues/442

[garettarrowood]

Travis passing for Ruby 2.2.2.