Regularly upload code coverage report to coveralls.io

Every time Travis CI builds this project it will also generate a Clover-style code coverage report and upload it to coveralls.io. For a sample of the end result, see https://coveralls.io/r/ycombinator/php-opencloud.

**Note**: Right *before* merging this PR, please turn on the integration between this GitHub repo and coveralls.io. To do this:
1. Please visit https://coveralls.io
2. Sign-in with the `rackspace` GitHub account.
3. Find/add this GitHub repository on this page: https://coveralls.io/repos/new.
4, Toggle the on/off switch to the on state.