Define extra dev dependencies in Gemfile.dev

Before this commit running `rake mutant` failed:

    /home/peter/.rvm/gems/ruby-2.1.5@global/gems/bundler-1.7.9/lib/bundler/rubygems_integration.rb:256:in
    `block in replace_gem': mutant is not part of the bundle. Add it to
    Gemfile. (Gem::LoadError)
      from /home/peter/.rvm/gems/ruby-2.1.5@pg/bin/mutant:22:in `<main>'

Usage:

    export BUNDLE_GEMFILE=Gemfile.dev
    bundle
    rake mutant