Remove Ubuntu Artful test (#127)

* Remove Ubuntu artful test

* Remove Artful from Travis build matrix

Author: alessfg

.travis.yml

@@ -168,34 +168,6 @@ matrix:
         distribution: ubuntu
         version: xenial
         playbook: unit
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: basic
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: http-template
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: modules
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: push
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: stable
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: stream-template
-    - env:
-        distribution: ubuntu
-        version: artful
-        playbook: unit
     - env:
         distribution: ubuntu
         version: bionic