Slack contact fixes for Ruby <1.9

These changes fix the issues mentioned in #172.

They were:

* A new-style symbol-as-key hash rather than the old hashrocket style
* Use of the `%` string formatter with a hash as the value argument, which has
  only existed since 1.9

All tests are now passing for me on 1.8.7-p374 and 1.9.3-p545.