pants: support macOS 10.13 ("High Sierra") (#2825)
pants: support macOS 10.13 ("High Sierra") (#2825)

- Map Darwin/17 to macOS 10.13
- Pants switched its binary repository from Bintray to S3. Use our mirror of that S3 repository.
(sapling split of ab844085afe18ca4d02cfde18ed7dc3487825c0e)
foursquare/fsqio,foursquare/fsqio,foursquare/fsqio,foursquare/fsqio,foursquare/fsqio