Remove traces of 1.8 support

Shoulda Matchers 2.0 removed support for 1.8. This change removes Ruby
1.8 tests from travis and sets the minimum Ruby version in the gemspec.
Ruby 2.0 Support

* 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.
Spelling fixes