[AIRFLOW-831] Restore import to fix broken tests

The global `models` object is used in the code and
was inadvertently
removed. This PR restores it

Closes apache#2050 from jlowin/fix-broken-tests

tests/jobs.py

@@ -26,7 +26,7 @@
 import sys
 from tempfile import mkdtemp
 
-from airflow import AirflowException, settings
+from airflow import AirflowException, settings, models
 from airflow.bin import cli
 from airflow.jobs import BackfillJob, SchedulerJob
 from airflow.models import DAG, DagModel, DagBag, DagRun, Pool, TaskInstance as TI