Specifiy path to pom.xml for maven-release-plugin

mvn release:perform wants to do a local checkout and expects to find the
pom.xml to execute in the base directory by default. Since we have several
Maven projects within one repo we have to tell it which one we are
executing.
Use a local checkout when doing mvn release:perform

A lot quicker than cloning the whole 1.1GB repo down from github each time.