Install the tidy command on Travis

Our conformance test didn't run correctly since we hadn't installed the
tidy command. Also update the contribution guideline with the install
command for few operating systems.

.travis.yml

@@ -1,3 +1,7 @@
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get -qq install tidy
+
 rvm:
   - 1.9.2
   - 1.9.3

CONTRIBUTING.md

@@ -7,19 +7,24 @@
    Set up one of these versions; use of [RVM][rvm] is recommended to switch
    easily between different versions.
 4. [Install bundler.][bundler]
-5. Check that unit tests pass with `rake test`.
-6. Write a failing test to capture existing bug or lack of feature.
-7. Run `rake test` to verify that test fails.
-8. Implement your feature or bug fix.
-9. Ensure tests pass.
-10. If it's a new feature or a bug fix,
-    please add an entry to the changelog file.
-11. Add, commit, and push your changes.
-12. [Submit a pull request.][pr]
-13. You will get some feedback and may need to push additional commits
+5. Make sure to have the `tidy` command on your system:
+
+   * `apt-get install tidy`
+   * `yum install tidy`
+   * `pacman -S tidyhtml`
+
+6. Check that unit tests pass with `rake test`.
+7. Write a failing test to capture existing bug or lack of feature.
+8. Run `rake test` to verify that test fails.
+9. Implement your feature or bug fix.
+10. Ensure tests pass.
+11. If it's a new feature or a bug fix, please add an entry to the changelog file.
+12. Add, commit, and push your changes.
+13. [Submit a pull request.][pr]
+14. You will get some feedback and may need to push additional commits
     with more fixes to the same branch; this will update your pull request
     automatically.
-14. [Squash your commits into a single one][rebase]
+15. [Squash your commits into a single one][rebase]
     and [force a push to the same branch][force_push].
 
 [branch]: http://git-scm.com/book/en/Git-Branching-Branching-Workflows#Topic-Branches