Disable runkit for PHP 5.6

It's currently broken, see https://github.com/zenovich/runkit/issues/69

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove plugin manager and observer classes

We don't really need the observer pattern, so let's simplify the code.

Signed-off-by: Michal Čihař <michal@cihar.com>