Updating CakePHPInstaller for upcoming version 3.0

For the upcoming CakePHP 3, the location of `cakephp-plugin` will be changed from `Plugin` to `plugins`.

This change enables this by overriding `getLocations()` and adjusting `CakePHPInstaller::$locations` according to the required `cakephp/cakephp` version.

In the `getLocations()` implementation, please let me know if there's a better way to get the required `cakephp/cakephp` Package object. And also, whether the version comparison code is correct/incorrect.

Also noticed that we never pass the `Composer` object when instantiating a new Installer object, so I add it in b072373.
I hope this works for non-stable releases like alpha, beta too.