Wait for a numeric on IRC notifications

Code originally by Nick Markwell (@duckinator), ported to the current
codebase by me. This is a port of the code in travis-ci/travis-core#56.
Remove unnecessary line

If we refer to @numeric_received when it's not initialized, we'll get
nil back, and since we just need a falsey value, that's enough.
Merge branch 'hh-irc-wait-for-numeric' of git://github.com/henrikhodne/travis-core into henrikhodne-hh-irc-wait-for-numeric
Add a timeout around wait_for_numeric.