Improvments

* Add `toolchain` option to pass in specific toolchain for building (#5864)
* Add several options to configure or disable `xcpretty` (#5751)
* Skip showing GitHub issues for build errors (#5610)
* Add Xcode 8 support: rename "OS X" to "macOS" (#5385)

https://github.com/fastlane-old/gym/releases/tag/1.8.0
