Disable velocity logs completely.

H/T @adriancole for the suggestion!

Fixes #151
I think the same effect could be obtained in a slightly cleaner way like this:
  velocityRuntimeInstance.setProperty(RuntimeConstants.RUNTIME_LOG_LOGSYSTEM, new NullLogChute());
where NullLogChute is org.apache.velocity.runtime.log.NullLogChute. I'd love for there to be a test here but it seems moderately hard to write one.