Code coverage

I've regenerated the travis.yml to enable code coverage for this plugin.
@tsteur @mattab I'm not sure how plugins should be tested atm. The autogenerated travis file only executes Plugin tests. As far as I have seen that, before "Core" tests were executed, aswell.
If any additional tests should be executed we should change that in the generation of travis files, an recreate the file here again.

Code-Coverage: https://coveralls.io/r/piwik/plugin-CustomAlerts