Fix stable-2.12 build and tests

Reference Gerrit plugin API 2.12.2 from maven central.

Upgrade the EasyMock version to fix this compilation error:
  PublisherExceptionTest.java:60: error: cannot find symbol
    captor = EasyMock.newCapture();

Change-Id: Iaa9e12f60a3e2537993c5ee0ed5ed57f608c2a29