[AIRFLOW-2835] Remove python-selinux (apache#3673)

This package is not used and it sometimes breaks the CI because it
is not available. Therefore it makes sense to just remove it :-)

Author: Fokko

.travis.yml

@@ -40,7 +40,6 @@ addons:
       - krb5-kdc
       - krb5-admin-server
       - oracle-java8-installer
-      - python-selinux
   postgresql: "9.2"
 python:
   - "2.7"