Rule compatible with old junit

- Mockito Rule based on MethodRule:
  - compatible with JUnit 4.7+, instead of 4.9+,
  - no need to initialize with ```new MockitoJUnitRule(this)``` - just ```new MockitoJUnitRule()```,
- minor: ugly fix for building Gradle in Windows (#97),
If possible I'd rather have this type inside `JUnitRuleTest` as it only serve this test case.