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
Rather than run `go get` at buildtime, it would be great to model these as `go_resource`s like mpdviz or terraform, using the commits specified in the gomfile. That way Homebrew can manage the downloads.