Carthage now requires Swift 3.1.

Fixed

• Worked around copy bug on APFS volumes (#2032). Thanks @BobElDevil!
• Improper cache invalidation with Xcode 9 (#1984). Thanks @BobElDevil!
• Trashing of folders in Carthage/Checkouts/ (#2027). Thanks @BobElDevil!
• Building for multiple SDKs with the new Xcode build system (#2053). Thanks @BobElDevil!
• Broken --log-path option (#1958). Thanks @michaelmcguire!
• Potential hang when unzipping invalid archives (#1997). Thanks @ikesyo!

Improved

• GitHub dependencies will be the same when specified with git (#1995). Thanks @blender!
• Building will now use checked out Cartfiles instead of getting them from Git (#2041). Thanks @BobElDevil!

Thank you to @ikesyo and @Dschee for improvements to the codebase! :tada: Thank you to @ikesyo, @mdiep, @jdhealy, and @BobElDevil for reviewing pull requests. :sparkles: