More parallelism = faster tests. Also better isolation for changes that
might only break on zsh or bash respectively.
This is defined via env variable, so someone running locally will have
all tests run sequentially as before.
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.
