Override JUnit version to 4.11 to maintain compatibility with Powermock

Powermock, which is used by Spring Security's tests, depends upon
internal JUnit classes which have changed incompatibly between 4.11
and 4.12. Therefore, to allow the Platform to build successfully, it's
necessary to override the version of JUnit provided by Spring Boot
1.2 (4.12) and reinstate the version of JUnit that was in Platform 1.0
(4.11).

Closes spring-io/platform#113