Add Node 12 to full build matrix on Travis

Having removed the ui test dependencies out of package.json
we can remove the 'allow failures' flag from the node 12 build.

Given how close Node 12 is to being LTS, we really need to pay
proper attention to it.

.travis.yml

@@ -9,5 +9,3 @@ matrix:
       before_script:
         - npm install -g istanbul coveralls
     - node_js: "8"
-  allow_failures:
-    - node_js: "12"