Swap out getEventManager() with eventManager()

Due to a recentish update to the Cake3 repo, the getEventManager method has been renamed to just eventManager in the base Controller class, causing all of Crud to fail with a missing method error. This updates Crud to use the new API.
See also: https://github.com/cakephp/cakephp/commit/9a3e0db66ccc13c7612c13c3a458c4584d4068d0
Merge pull request #190 from sikker/cake3

Swap out getEventManager() with eventManager()