Gem Bumped to 3.1.2

We've recently bumped our rails gem with the below fixes.
https://rubygems.org/gems/airbrake .

Version 3.1.2 - 2012-06-23 02:27:01 +0200
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Hrvoje Šimić (5):
routing errors ignored by default, for #92
use simplecov for test coverage
fix features for javascript
ignore user agents in rack, closes #61
error out if controller is undefined