Group gems for leaner dependency resolution

See http://docs.travis-ci.com/user/languages/ruby/#Speed-up-your-build-by-excluding-non-essential-dependencies

~~This currently fails because ExceptionNotification is configured in the `test` rails environment. I don't think we need this while running tests?~~

Looks like the ExceptionNotification is required in test because we're doing something other than default behaviour and testing it.

Might help a tiny bit with #762 

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