Add continuous integration tests

[stweil]

Automated builds with error reporting would be helpful. Maybe there are also tools for static code analysis which can be applied here.

[stweil]

Useful links:

• https://github.com/integrations/feature/continuous-integration
• https://docs.travis-ci.com/user/multi-os/
• https://docs.travis-ci.com/user/osx-ci-environment/
• https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/testing_with_xcode/chapters/08-automation.html
• https://developer.apple.com/library/content/technotes/tn2339/index.html#//apple_ref/doc/uid/DTS40014588-CH1-HOW_DO_I_BUILD_MY_PROJECTS_FROM_THE_COMMAND_LINE