Update Contributing link to avoid 404

[luilver]

Before submitting a pull request, please make sure the following is done:

• [x] Fork the repository and create your branch from master.
• [ ] If you've added code that should be tested, add tests!
• [ ] Ensure the test suite passes.
• [ ] Format your code with ShellCheck.
• [x] Include a human-readable description of what the pull request is trying to accomplish.
• [ ] Steps for the reviewer(s) on how they can manually QA the changes.
• [x] Have a fixes #XX reference to the issue that this pull request fixes.

[inkarkat]

@karbassi I cannot merge this pull request, not even outside GitHub; it won't let me push because the travis-ci step hasn't run yet. This step is deprecated, right? I think it now should be removed (or repaired).