Attempt to kickstart Travis CI. (GoogleCloudPlatform#1733)

* Attempt to kickstart Travis CI.

* Reintroduce Airflow changes

Author: frankyn

.travis.yml

@@ -16,7 +16,7 @@ env:
     # package installation continue. See:
     # https://github.com/apache/incubator-airflow/pull/3660
     - SLUGIFY_USES_TEXT_UNIDECODE=yes
-    secure: fsBH64/WqTe7lRcn4awZU7q6+euS/LHgMq2Ee2ubaoxUei2GbK5jBgnGHxOKVL5sZ4KNfTc7d6NR5BB1ZouYr2v4q1ip7Il9kFG4g5qV4cIXzHusXkrjvIzQLupNpcD9JJZr1fmYh4AqXRs2kP/nZqb7xB6Jm/O+h+aeC1bhhBg=
+    - secure: fsBH64/WqTe7lRcn4awZU7q6+euS/LHgMq2Ee2ubaoxUei2GbK5jBgnGHxOKVL5sZ4KNfTc7d6NR5BB1ZouYr2v4q1ip7Il9kFG4g5qV4cIXzHusXkrjvIzQLupNpcD9JJZr1fmYh4AqXRs2kP/nZqb7xB6Jm/O+h+aeC1bhhBg=
 addons:
   apt:
     sources: