fix update check

fixed version check on update, also abort if passed a nonexistent version and added usage docs.
Merge branch 'master' of https://github.com/hlarsen/boxcar into hlarsen-master

Conflicts:
	lib/boxcar/command/update.rb
Refactor update command
Fix all_versions check
Remove boot old boot script reference
Merge branch 'master' into hlarsen-master
Merge remote-tracking branch 'github/hlarsen-master'