Use debugger 1.6.0.

This version is compatible with both 1.9.3-p448 and 2.0.0-p247.
Change assert_match parameters order.

This way we please both ruby 1.9 test framework and mini test for
ruby 2.0.

Thanks to Chi-Ting Leung <chiting.leung@gmail.com>
Merge branch 'topic/ruby-2-compat'

* topic/ruby-2-compat:
  Change assert_match parameters order.
  Use debugger 1.6.0.