ActivitiRule services not initialized when using SpringJUnit4ClassRunner together with @ContextConfiguration

When using SpringJUnit4ClassRunner together with @ContextConfiguration
to inject a configured ProcessEngine into the ActivitiRule, then you
can't use any of the ActivitiRule.get*Service() methods since they all return null.

The problem is that initializeServices() is not called in the "starting(FrameworkMethod method)" method since
it is only called when the processEngine is null.

A possible fix would be to call initializeServices() in setProcessEngine.