Make reek run warning-free

Enables warnings during the spec run, and fixes the following warnings:

* 'circular require considered harmful' due to some needless requires
* 'copying extra states' warning when using Hash#reject
* 'character class has duplicated range' warning due to using match
  matcher with a string argument
This was to make Ruby stop emitting 'copying extra states' warnings. Very odd.

I also think it's more readable, though :-).