making travis setup more generic

Let travis use a standard script, which can be reused in any plugin.

Required steps for reuse:
* Put this travis.sh in the root of your plugin
* Put this .travis.yml in the root of your plugin
* update REPO_NAME, to match the github repo name
* update PLUGIN_NAME to match CamelCased plugin naming for CakePHP
* ensure the plugin has an All{PluginNameHere}Test.php