fix: do not send deployment notification in dry-run mode

Without this diff, a successful deployment notification is sent to the flow even if Capistrano runs in dry_run mode (the -n flag). 

I'm proposing this fix because using ``capistrano-slack`` without it can cause great confusion in a team, leading people to believe that a new version has been deployed when it was not.

PS: Sorry about the messy diff, I took this opportunity to streamline the code indentations (there was a mix of 2 and 4 spaces before). If this is a problem, I can submit a new pull request with only the actual code change.