Coveralls support

I have added support for coveralls to the omnipay module (via the [php-coveralls](https://github.com/satooshi/php-coveralls) library), however it required a change to the generated composer file, as specified in this pull request. I'm not sure if it is desirable to have this addition to silverstripe travis support.

I did originally try doing the extra composer require with a extra travis command, but i got the message: [Package "satooshi/php-coveralls" listed for update is not installed. Ignoring.](https://travis-ci.org/burnbright/silverstripe-omnipay/builds/18163573#L106)