fix minitest warning

This deals with the following warning (seen with Ruby 2.1.2):

    Warning: you should require 'minitest/autorun' instead.
    Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
    From:
      /Users/christian/.rbenv/versions/2.1.2/lib/ruby/2.1.0/test/unit.rb:1:in `require'
      /Users/christian/.rbenv/versions/2.1.2/lib/ruby/2.1.0/test/unit.rb:1:in `<top (required)>'
      /Users/christian/Projects/calabash-ios/calabash-cucumber/lib/calabash-cucumber/operations.rb:9:in `require'