ENH: Add parallelism to CircleCI builds
ENH: Add parallelism to CircleCI builds

This appears to halve the time it takes to build.

Author: Phillip Cloud <cpcloud@gmail.com>

Closes #958 from cpcloud/ci-parallelism and squashes the following commits:

1789c20 [Phillip Cloud] Add parallelism to CircleCI builds

wesm/ibis,wesm/ibis,ibis-project/ibis,cpcloud/ibis,cloudera/ibis,cloudera/ibis,ibis-project/ibis,deepfield/ibis,deepfield/ibis,deepfield/ibis,cpcloud/ibis,deepfield/ibis,wesm/ibis,cpcloud/ibis,cpcloud/ibis,cloudera/ibis,ibis-project/ibis,ibis-project/ibis