Don't gather coverage data for spec files

* until now simplecov simply gathered coverage data for ALL files
  that were loaded, this includes spec files where of course the
  coverage was 100%
* this, well is not exactly the goal, as we want to gather
  coverage data on our implementation.
* the behavior was nice though to spot shared samples that were
  not used/unexecuted test but it's not the main functionality
* This of course drops our test coverage by a good 7% I belive
  (94% --> 87%) and coveralls will complain about that any second
  but this is more honest and accurate :-)
Merge pull request #372 from PragTob/simplecov_spec_filter

Don't gather coverage data for spec files