These versions are still not working properly: https://travis-ci.org/nlohmann/json/jobs/251675281

Maybe the simple approach is working now - at least it is in https://github.com/taocpp/PEGTL/blob/master/.travis.yml

See https://twitter.com/travisci/status/880550780368855040

Also added a Travis builder with -std=c++1z

Taken from
https://github.com/travis-ci/docs-travis-ci-com/pull/746/files.

https://blog.travis-ci.com/2017-03-28-xcode-83-ga-is-here

This test case relied on logics that have been replaced by CMake with
#461. This change enables compilation and execution of the test suite
without exceptions by adding an after_success task.

- get CMake for XCode 8.1 image

Conflicts:
	.travis.yml
	Makefile

See https://travis-ci.org/nlohmann/json/jobs/183684093 for a failing
build.