DDF-475 Move to PowerMock 1.5.4, for compatibility with Mockito 1.9.5

This is an indirect result of needing to upgrade Mokito for compatibility with
PowerMock 1.5.4, which was required over in ddf-catalog for Oracle JDK 8 compatibility.
Merge pull request #4 from bradh/DDF-475

DDF-475 Move to PowerMock 1.5.4, for compatibility with Mockito 1.9.5
Reviewed by: @mrmateo