Enable tracing in hooks when DOKKU_TRACE is set.

This makes it much easier to debug plugins.
Also source the 'dokkurc' file when starting dokku. This makes it easy
to define DOKKU_TRACE over ssh.
Enable DOKKU_TRACE when running travis tests.
Merge pull request #317 from plietar/master

Enable tracing in hooks when DOKKU_TRACE is set.