Add Treadle to CI builds (#1516)

Change CI to build Treadle master from source. This fixes a bug where
Treadle would be pulled from the latest snapshot (which would pull the
latest FIRRTL snapshot). For bleeding edge PRs that rely on new FIRRTL
features, using a FIRRTL snapshot may result in compilation failures.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>