Put mutant inside an gemfile mri platform group

* Platform :ruby_21 is not known by old rubies.
* Devtools only supports 1.9+ so no the :mri group is a better
  selector.
Correct syntax of prior commit
Add mri-2.1.0 to mutant enabled platforms
Merge remote-tracking branch 'upstream/master'
Change `Config::Yardstick#options` to skip `nil` values.

Allows `yardstick` to correctly set defaults for options not
specified in `config/yardstick.yml`
Merge branch 'no-nils-from-yardstick-config'