Update outdated tests (#6515)

These tests were running on Ubuntu Precise and Debian Wheezy which have reached their end of life and are no longer maintained by the respective distros. This updates the tests to a newer version of Debian and Ubuntu.

* Remove tests on the deprecated precise.

* Add tests for Xenial.

* update Jessie tests to use Wheezy

* update .travis.yml

tox.ini

@@ -198,17 +198,6 @@ whitelist_externals =
 passenv =
     DOCKER_*
 
-[testenv:le_auto_precise]
-# At the moment, this tests under Python 2.7 only, as only that version is
-# readily available on the Precise Docker image.
-commands =
-    docker build -f letsencrypt-auto-source/Dockerfile.precise -t lea letsencrypt-auto-source
-    docker run --rm -t -i lea
-whitelist_externals =
-    docker
-passenv =
-    DOCKER_*
-
 [testenv:le_auto_trusty]
 # At the moment, this tests under Python 2.7 only, as only that version is
 # readily available on the Trusty Docker image.
@@ -222,11 +211,20 @@ passenv =
     DOCKER_*
     TRAVIS_BRANCH
 
-[testenv:le_auto_wheezy]
+[testenv:le_auto_xenial]
+# At the moment, this tests under Python 2.7 only.
+commands =
+    docker build -f letsencrypt-auto-source/Dockerfile.xenial -t lea letsencrypt-auto-source
+    docker run --rm -t -i lea
+whitelist_externals =
+    docker
+passenv = DOCKER_*
+
+[testenv:le_auto_jessie]
 # At the moment, this tests under Python 2.7 only, as only that version is
 # readily available on the Wheezy Docker image.
 commands =
-    docker build -f letsencrypt-auto-source/Dockerfile.wheezy -t lea letsencrypt-auto-source
+    docker build -f letsencrypt-auto-source/Dockerfile.jessie -t lea letsencrypt-auto-source
     docker run --rm -t -i lea
 whitelist_externals =
     docker