Don't run in parallel while building site in CI
Don't run in parallel while building site in CI

This will make it slower, but in the new circleci v2 world I was seeing a lot
of failures because one (or more) of the parallel workers used to build the site
were failing to load the middleman command.

unboxed/unboxed.co,unboxed/ubxd_web_refresh,unboxed/unboxed.co,unboxed/ubxd_web_refresh,unboxed/ubxd_web_refresh,unboxed/unboxed.co