Zef fails to install JSON::Fast when running in Travis with Docker
It should work if you try it again / zef update. JSON::Fast is actually in the cpan ecosystem, but the cpan ecosystem itself has been having issues the last couple of days.
The p6c ecosystem not updating is an unrelated issue (since JSON::Fast is in the cpan ecosystem). That can be corrected by updating zef -- zef install https://github.com/ugexe/zef.git