Merge pull request #18 from cthomaspdx/master

Allow for custom notifications and messages.
Fix for issue

:slack_emoji is mandatory #9
may as well fix this while I am at it.
Keep from failing deploy if slack is down.
Update slack.rb
Merge pull request #1 from cthomaspdx/new_master

HHWEB-4091  Capistrano > Slack Integration > Breaks deploy if slack is down