Version updates

This change updates the versions of all dependencies.
New Relic Agent Upgrade

This change upgrades the buildpack to use the 3.7.x line of the New
Relic agent.
Empty classpath improvements

Previously, when there were no additional libraries, the classpath
would render as '-cp '.  This was a problem when that argument was
followed by something else.  Applications would end up with something
like '-cp  app.groovy' which was incorrect.  This change updates the
as_classpath() method to return nil if there is are no additional
libraries ensuring that the '-cp' argument is never added to the
command.