Adds the dinghy tap before trying to install it.
Test Plan:
1. On a machine running OSX without dinghy installed, run the
docker_dev_setup script. Verify dinghy is set up without issue.
Change-Id: I0479323aefd21554fb3bc2a8f00ced727b497496
Reviewed-on: https://gerrit.instructure.com/132850
Reviewed-by: Omar Khan <okhan@instructure.com>
Tested-by: Jenkins
QA-Review: Omar Khan <okhan@instructure.com>
Product-Review: Omar Khan <okhan@instructure.com>