Release

* Add support for `imagick` extension
* Frameworks now support a `post-compile` method
* `composer.lock` is now mandatory
* Composer packages are now detected by looking at the `composer.lock`.
* Vulcan is now installed with bundler
* A `HEROKU_BUILD_TIME` variable is now set when compiling the slug, which is available at runtime.
* Symfony apps now only expose `app.php`, previously also `app_dev.php` was reachable.