Turn parallel_tests back on for CI

[kpaulisse]

Recently the CI for octocatalog-diff has been pretty flaky due to random tests getting killed. From travis CI documents, I concluded that the individual tests may be exhausting resources of the container, and as such in #161 I disabled the parallel_tests gem for CI. Since then, no problems. 🤞 But, unfortunately the CI build time per ruby version increased by about 5 minutes (20 to 25) as a result of this change.

I'm entering this issue to keep track turning this back on. Things that would probably need to be done here would be to evaluate whether the tests can be effectively parallelized (perhaps the spec tests) or if the number of simultaneous processes can be reduced to keep from bumping up against any limits that exist.

Right now nobody is actively working on this (there are higher priorities). However if test parallelization is something that interests you and you'd like to have a go at making the CI faster, please comment in the issue! 😸