Fixes to Slack contact for Ruby <1.9

This fixes:

- One instance of the new hash syntax rather than the old hashrocket
syntax
- A use of the `%` format for strings that used a hash for arguments;
this was introduced in Ruby 1.9
Revert "Revert "Merge pull request #172 from robmiller/feature/contacts/slack""

This reverts commit 47ee272b02fb3f694d6c775f42709036847fa818.
Merge branch 'feature/contacts/slack' of https://github.com/robmiller/god into slack-hook
Escape curly braces in 1.8