SpringProcessEngineTestCase#clearTestContextCache() throws a NoSuchFieldException

With Spring 4 the class TestContextManager does not contain the field contextCache anymore. So whenever a spring test case is executed with Spring 4 a NoSuchFieldException is thrown.

AT:

• provide an if-clause (or try/catch) whether the field contextCache does exist

Hints:
https://github.com/camunda/camunda-bpm-platform/blob/master/engine-spring/src/main/java/org/camunda/bpm/engine/spring/impl/test/SpringProcessEngineTestCase.java#L64