Add to homebrew?

[AndrewSB]

Doing

- curl -L -O https://github.com/Carthage/Carthage/releases/download/0.16.2/Carthage.pkg
- sudo installer -pkg Carthage.pkg -target /
- curl -L -O https://github.com/YPlan/CartfileDiff/releases/download/0.1/CartfileDiff.pkg
- sudo installer -pkg CartfileDiff.pkg -target /
install: true
script: script/cibuild

for every build is fun, but I think it would be a lot easier if carthagediff could be vended through homebrew!