Updating maven-release-plugin to 2.5.1

There is a bad interaction between the old version of the maven-release-plugin and a new version of git.
https://jira.codehaus.org/browse/MRELEASE-812

Basically, the "prepare" stage would fail to push the updated poms, resulting in the release version still being a "SNAPSHOT." Version 2.5.1 of the maven-release-plugin remedies this.