Switch to the Packagist api for discovery and general cleanup

Fixes #42

Changes made:
* Updates the composer.lock file and switch the install instructions to --no-dev
* Uses the Packagist search API to get the names of silverstripe packages and then use a repository pool to load the information about that package
* Remove [Pretty]Alias, as it's no longer provided. PrettyVersion seems to work in its place.
* clear when running AddonUpdater now deletes all Addon classes and the cascades clean up the relations better (has_manys are deleted, all many_many information is cleared).