Update outdated tests (#6515)

These tests were running on Ubuntu Precise and Debian Wheezy which have reached their end of life and are no longer maintained by the respective distros.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

letsencrypt-auto-source/Dockerfile.jessie

@@ -1,7 +1,7 @@
 # For running tests, build a docker image with a passwordless sudo and a trust
 # store we can manipulate.
 
-FROM debian:wheezy
+FROM debian:jessie
 
 # Add an unprivileged user:
 RUN useradd --create-home --home-dir /home/lea --shell /bin/bash --groups sudo --uid 1000 lea

letsencrypt-auto-source/Dockerfile.xenial

@@ -1,7 +1,7 @@
 # For running tests, build a docker image with a passwordless sudo and a trust
 # store we can manipulate.
 
-FROM ubuntu:precise
+FROM ubuntu:xenial
 
 # Add an unprivileged user:
 RUN useradd --create-home --home-dir /home/lea --shell /bin/bash --groups sudo --uid 1000 lea