Remove Devtools [WIP]

@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
  * [x] Use Version 2.99 to be informed about deprecations
  * [x] 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.
* [ ] Run Reek, Rubocop and Mutant (mutant and mutant-rspec) optionally?
* [ ] 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
* Look through Yard Errors and Warnings
* Look through Inch Report