Set up RuboCop for this repo

[mattbrictson]

Given that raygun generates Rails projects that have an opinionated RuboCop config, it is a bit surprising that raygun itself does not use RuboCop. In the interest of dog-fooding we should add our standard c5-conventions RuboCop config to this project.

It would also be nice to be able to check out this project and run:

$ bundle exec rake

And the default rake task would run RuboCop (and eventually, tests).

Likewise if/when we set up CI, a RuboCop job should be part of it.

[mattbrictson]

Fixed in #152 and #148