Use Ember v3.12.0

Bumped JS dependencies where possible

gui/.travis.yml

@@ -1,7 +1,7 @@
 ---
 language: node_js
 node_js:
-  - "6"
+  - "8"
 
 sudo: false
 dist: trusty
@@ -17,13 +17,6 @@ env:
     # See https://git.io/vdao3 for details.
     - JOBS=1
 
-before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash
-  - export PATH=$HOME/.yarn/bin:$PATH
-
-install:
-  - yarn install --non-interactive
-
 script:
   - npm run lint:hbs
   - npm run lint:js