Remove Devtools

@railsbros-dirk and I discussed this for a while: Devtools costs us a lot of time and doesn't provide the according value. Therefore we will remove it.

* [x] Remove Devtools
* [x] Add Rake
* [x] Add RSpec
    * Use Version 2.99 to be informed about deprecations
    * Add according Rake task
* [x] Code Coverage via CodeClimate Coverage Reporter on Travis (remove Coveralls and local run)
* [x] Remove Flay and Flog – they run on Code Climate
* [x] Re-add YARD and the YARD Rake Task
* [x] Remove Yardstick, replace it with Inch. Add the according Raketask.
* [x] Run Reek
* [x] Run Rubocop
* [x] Run Mutant (Gems: mutant and mutant-rspec)
* [x] [The 7 lines every Rakefile should have](http://erniemiller.org/2014/02/05/7-lines-every-gems-rakefile-should-have)
* [x] Define a sensible task hierarchy. Maybe include a `metrics` group again?
* [x] Define a sensible `rake ci` task. Currently it only runs the test suite
* [x] Add Guard and its configuration

After this pull request is merged, the following things will be done in separate pull requests:

* Look through RSpec Deprecation Warnings #94 
* Look through Yard Errors and Warnings #95
* Look through Inch Report #95
* Fix RuboCop warnings and add Rubocop to the `ci` task #96
* Run mutant in a separate travis job #97
* Run Rubocop guard in the 'refactor' part of red-green-refactor #98