Adding instructions on how to get guard running
Conditionally include older version of terminal-notifier-guard for Mac OS 10.8

"As of version 1.6.1, we no longer bundle notifiers binaries in this
gem. Please revert to version 1.5.3 for OSX 10.8 support."
https://github.com/Springest/terminal-notifier-guard#osx-108-users
Merge pull request #2 from ndlib/adding-guard-instructions

Adding guard instructions