tests: Attempt to build for trusty and xenial both

.travis.yml

@@ -1,6 +1,8 @@
 sudo: required
 language: python
-dist: trusty
+dist:
+  - trusty
+  - xenial
 python:
   - "2.7"
 after_success:

certidude/cli.py

@@ -730,8 +730,8 @@ def certidude_setup_authority(username, kerberos_keytab, nginx_config, country,
     # Install only rarely changing stuff from OS package management
     apt("python-setproctitle cython python-dev libkrb5-dev libffi-dev libssl-dev")
     apt("python-mimeparse python-markdown python-xattr python-jinja2 python-cffi")
-    apt("python-ldap python-openssl software-properties-common libsasl2-modules-gssapi-mit")
-    pip("gssapi falcon cryptography humanize ipaddress simplepam humanize requests")
+    apt("python-ldap software-properties-common libsasl2-modules-gssapi-mit")
+    pip("gssapi falcon cryptography humanize ipaddress simplepam humanize requests pyopenssl")
     click.echo("Software dependencies installed")
 
     if not os.path.exists("/etc/apt/sources.list.d/nginx-stable-trusty.list"):