allow ruby-head to fail travis builds
Merge pull request #268 from codekitchen/allow-ruby-head-failures

allow ruby-head to fail travis builds
implement timed backoff on server failures

This is based on raven-python's implementation, with some tweaks for
ruby idioms.

With this commit, if we don't send, we only log the event id. I'd like
to extend this to log the entire event, along with also logging the
entire event when a sending failure occurs.
log event message on send failure or skip