Integrate with a CI system

[lloeki]

Travis and CircleCI both provide macOS builders.

Goals:

• [x] set up a non-tainted, pristine macOS environment
• [x] build packages from commits on master
• [x] build packages from branches
• [x] build multiple packages in dependency order
• [x] push new packages to repository when branch merged into master
• [x] push updated packages to repository when branch merged into master
• [x] remove deleted packages from repository when branch merged into master
• [ ] build snapshot on demand (tag?)
• [X] update repositories in an isolated step
• [ ] sign packages in an isolated step
• [ ] sign repositories in an isolated step