Get le-auto tests running on Travis.

tox.ini

@@ -75,3 +75,13 @@ setenv =
 commands =
     pip install -e acme -e .[dev] -e letsencrypt-apache -e letsencrypt-nginx -e letsencrypt-compatibility-test -e letshelp-letsencrypt
     sudo ./letsencrypt-apache/letsencrypt_apache/tests/apache-conf-files/apache-conf-test --debian-modules
+
+[testenv:le_auto]
+# At the moment, this tests under Python 2.7 only, as only that version is
+# readily available on the Trusty Docker image.
+commands =
+    docker build -t lea letsencrypt-auto-source
+    docker run --rm -t -i lea
+whitelist_externals =
+    docker
+passenv = DOCKER_*