Bring role up to spec (2.5+, Debian 9) and introduce LetsEncrypt support (#5)

* Bring role up to spec (2.5+) and introduce LetsEncrypt support
* Add Debian 9 support and fix script args chdir
* Use correct schema for travis and use command instead of script
* Make use of requirements.txt for build and fix syntax
* Got better at reading the docs now
* Minor improvements

meta/main.yml

@@ -4,7 +4,7 @@ galaxy_info:
   description: Deploy DirectAdmin with Ansible
   company: Hyperized Hosting
   license: MIT
-  min_ansible_version: 2.1
+  min_ansible_version: 2.5
   platforms:
   - name: EL
     versions:
@@ -17,6 +17,7 @@ galaxy_info:
     - jessie
     - squeeze
     - wheezy
+    - stretch
   galaxy_tags:
   - system
   - web