Travis updated the container images so that the perl
reference to 5.18 was outdated. We use now 5.26 which
works, however we should consider to be more flexible.
JSON::Validator didn't compile in the container. Thus
we switched to just use 'JSON'. That also supports JSON
pretty. For the future we should just test for valid JSON
in all unit test files as it is more effective.
Dirk Wetter
Frank Breedijk