Update to current LTS version, send SNS notification for all build return types, allow to specify template for subject and message notification.

From the fork history of the original snsnotify-plugin it seems like Julian's (@jkelabora) branch is the most active one, therefore I used this as a base. The change I propose includes variable substitution (build and environment variables) for subject and message to allow customization of what is being posted as SNS notification, it is backward compatible (so in case you leave the values empty it will behave like before).

Sorry that this pull request contains some more commits at once.
@mikewatt please review and let us know what you think.