Ensure that we `rake dist` prior to running tests.

This fixes an issue that occurs on Travis after we checkout a new
branch. Previously, the builds would timeout due to CPU contention with
`phantomjs`, `defeautureify`, and `puma` all running at once (Travis
only provides 1.5 CPU's).
Merge pull request #108 from rjackson/ensure_build_prior_to_tests

Ensure that we `rake dist` prior to running tests.