Brings JPile under the opower-parent umbrella of maven plugin management

Now that opower-parent:2.0.0 is out on maven-central, we can centralize some of our build configurations (e.g. Java version, checkstyle version, etc) amongst our Opower-managed projects.

This commit defers version management of its maven plugins up to opower-parent and removes now-redundant pom sections that our parent controls.

In the act of deferring our checkstyle version up to pose-parent, we're affecting an upgrade that removes the support for the DoubleCheckLocking check, so that's been removed from jpile's checkstyle.xml