Run le_auto_xenial on every PR. (#7195)

https://github.com/certbot/certbot/pull/7190/files removed our only le_auto_* tests on PRs. This PR fixes that by running le_auto_xenial on every PR which also includes running modification-check.py like we used to for Trusty.

.travis.yml

@@ -72,6 +72,10 @@ matrix:
       before_install:
       addons:
       <<: *not-on-master
+    - sudo: required
+      env: TOXENV=le_auto_xenial
+      services: docker
+      <<: *not-on-master
     - python: "2.7"
       env: TOXENV=apacheconftest-with-pebble
       sudo: required
@@ -205,10 +209,6 @@ matrix:
       sudo: required
       services: docker
       <<: *extended-test-suite
-    - sudo: required
-      env: TOXENV=le_auto_xenial
-      services: docker
-      <<: *extended-test-suite
     - sudo: required
       env: TOXENV=le_auto_jessie
       services: docker