Ruby 2.0 Support

* Fix assigns_matcher spec: Ruby 2.0 no longer calls `to_s` on
 `inspect`. This changes this matchers failure message slightly in 2.0,
  but it's still readable and correct.
* Fix delegate_matcher: Under 2.0.0, the unexpected message to the mock
  is throwing a `MiniTest::Assertion` error. Add that to the list of
  exceptions we catch and trigger failure on.
* Add Ruby 2.0 support to the Appraisals
* Update travis to test 2.0.0 with the 3.2 gemfile. Clean up the travis
  matrix to favor the include syntax over exclude.
* Edit gemfile.locks to point to relative paths.