Add Formula for aptly 0.7.1

Summary:
Add Formula for aptly 0.7.1. Aptly is an apt repository package manager.

Test Plan:
brew install aptly
aptly version
brew uninstall aptly
Can a test be added to do something more substantial then e.g. `--version` or `--help`? See `cmake.rb` for an example of a formula with a good test. Thanks!