Remove requirement for terminal-notifier cli tool

[voltechs]

Since there is a gem that does this stuff, it might be worthwhile to leverage it, versus through the command line, and subsequently requiring users to install the dependency outside of this gem.

https://github.com/julienXX/terminal-notifier/tree/master/Ruby

For now, I suppose I will just use that gem, versus this one, but I love the "universalness" of this gem.