Add `terminal-notifier` runtime dependency

[mobilutz]

I had this problem Gem::Exception: can't find executable terminal-notifier.

I am using rbenv and could not figure out why running terminal-notifier failed even when running in the shell.

After a long search I found the issue that the gem terminal-notifier was not in the Gemfile.lock. I added it manually to Gemfile but adding this dependency should fix it in a better way.

[mobilutz]

Been such a long time, I will close it.