Remove devtools

This has already be done for [Ashikawa](https://github.com/triAGENS/ashikawa-core/pull/93) and we're doing well over there. Thus we will remove it as well ;-)

* [ ] Remove Devtools
* [ ] Add Rake
* [ ] Add RSpec
    * Use Version 3.0
    * Fix the errors
    * Add according Rake task
* [ ] Code Coverage via CodeClimate Coverage Reporter on Travis (remove Coveralls and local run)
* [ ] Remove Flay and Flog – they run on Code Climate
* [ ] Re-add YARD and the YARD Rake Task
* [ ] Remove Yardstick, replace it with Inch. Add the according Raketask.
* [ ] Run Reek
* [ ] [The 7 lines every Rakefile should have](http://erniemiller.org/2014/02/05/7-lines-every-gems-rakefile-should-have)
* [ ] Define a sensible task hierarchy. Maybe include a `metrics` group again?
* [ ] Define a sensible `rake ci` task. Currently it only runs the test suite
* [ ] Add Guard and its configuration

The warnings that Inch reported will be addressed in a [separate pull-request](https://github.com/triAGENS/guacamole/issues/76).


Prefer single-quoted strings when you don't need string interpolation or special symbols.