consider adding travis-ci integration

[danmarsden]

The checks run in the moodle.org plugins db can also be run automatically here on github using the travis-ci integration.

More information on this is here:
https://moodle.org/mod/forum/discuss.php?d=323384
this will pick up a number of other things that we would normally notice in the review process.

Integrating with travis is optional and will not block approval in the plugins db, but it does make the review process a bit easier and usually provides a good way to ensure future compliance with coding guidelines etc.