The travis config for Carthage seems outdated, resulting in the following error:
carthage: A full installation of Xcode.app 9.0 is required to compile this software.
Installing just the Command Line Tools is not sufficient.
Xcode 9.0 cannot be installed on macOS 10.11.
You must upgrade your version of macOS.
I would be tempted by replacing osx_image: xcode7.3 by osx_image: xcode9 in the travis config file
The travis config for Carthage seems outdated, resulting in the following error:
I would be tempted by replacing osx_image: xcode7.3 by osx_image: xcode9 in the travis config file