don’t bother on macosx (preinstalled) or if the test matrix for that
box isn’t going to be testing in zsh.
this should speed up test runs for most cases, and later we can define
fast_failure and not have to wait for the longer ones.
Requires beta access, thankfully the travis gods have blessed us for the
main repo and my fork!
This should allow us to make sure scm_breeze operates reliably in both
Linux and BSD/Darwin, because there are small shell differences
(especially with default tools) that are causing errors I noticed on
MacOSX.
