Xcode 12 beta 6 and Xcode 11.7 now available

Xcode 11.7 (11E801a) build environment is now available! Just add osx_image: xcode11.7 to your .travis.yml file to run your jobs on this version.

We also updated the Xcode 12 image, now running the beta 6 (12A8189n). You can use the same tag, osx_image: xcode12, in your .travis.yml file. In both environments, Fastlane was updated to version 2.157.2 and SwiftLint to 0.40.1.