All classes from the org.activiti.engine.impl.pvm package (and subpackages) have been removed. This is because the PVM (Process Virtual Machine) model has been removed and replaced by a simpler and more lightweight model.
This means that usages of ActivitiImpl, ProcessDefinitionImpl, ExecutionImpl, TransitionImpl are invalid.
For all classes related to activiti-rest, I suggest reading https://www.activiti.org/userguide/#_rest_api as they're not using Restlet anymore. SpringMVC is now used with an easy integration with SpringBoot, and Spring in general.