Travis support

Already in place for the Ruby client, makes sense for PHP as well :) You'll need to activate the project in Travis after merging this, by logging into Travis via Github on an account that has admin privileges on this repo.

Fixed up the `require_once` call in the PHPUnit bootstrap to use forward slashes for directory separation, since its compatible with both Unix and Windows. That seems to be in place for other `require` calls across the codebase already, and might've been missed in this case.

Verified the build passes on my fork: https://travis-ci.org/chillu/raygun4php/builds/28091609