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 ;-)

* [x] Remove Devtools
* [x] Add Rake
* [x] Add RSpec
    * Use Version 3.0
    * Fix the errors
    * 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] [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

The warnings that Inch reported will be addressed in a [separate pull-request](https://github.com/triAGENS/guacamole/issues/76).