Nuke rubocop
Add Kernel#send => Kernel#public_send mutation

Closes #78
Emit mutator from Object#gsub => Object#sub

Closes #85
Update metrics thresholds
Upgrade gem dependencies

* Add rom-rb/rubocop back
* Update yardstick to use git repository instead of gem
Add missing YARD docs
Add nthref mutator

Closes #20
Bump parser dependency to ~> 2.0.0.pre6
Readd -r / --require option

Closes #58
Add Mutant::Version and --version flag for CLI

Closes #88
Merge branch 'master' into flexible-rspec
Initial rspec killer with "longes description prefix match"