Parse the slack url. Reject invalid urls.
Merge pull request #79 from librato/fix/slack-uri

Parse the slack url. Reject invalid urls.
Version bump to 2.20.0
Regenerate gemspec for version 2.20.0
Error if url is blank, not just non-nil
Version bump to 2.21.0
Regenerate gemspec for version 2.21.0
Merge branch 'master' into feature/slack-clearing
Adjust slack service based on feedback from slack