travis-ci: specify IRC server/channel via environment variable configured on travis-ci.org.

This is to prevent our IRC dev channel from being notified when other people run BP's unit tests with travis-ci, via their own Github forks.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9080 cdf35c40-ae34-48e0-9cc9-0c9da1808c22