Use Yarn instead of outdated npm

.travis.yml

@@ -1,27 +1,30 @@
 ---
 language: node_js
 node_js:
-  - "6"
+  # we recommend testing addons with the same minimum supported node version as Ember CLI
+  # so that your addon works for all apps
+  - "4"
 
 sudo: false
 
 cache:
+  yarn: true
   directories:
-    - $HOME/.npm
     - $HOME/.cache # includes bowers cache
 
 before_install:
-  - npm config set spin false
-  - npm install -g bower phantomjs-prebuilt
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH=$HOME/.yarn/bin:$PATH
+  - yarn global add bower phantomjs-prebuilt
   - bower --version
   - phantomjs --version
 
 install:
-  - npm install
+  - yarn install --no-lockfile
   - bower install
 
 script:
-  - node_modules/.bin/ember try:each
+  - yarn run ember try:each
 
 deploy:
   provider: npm

ember-cli-build.js

@@ -1,3 +1,5 @@
+'use strict';
+
 /* jshint node:true*/
 /* global require, module */
 const EmberAddon = require('ember-cli/lib/broccoli/ember-addon');

hbs-minifier-plugin.js

@@ -1,3 +1,5 @@
+'use strict';
+
 /* eslint-env node */
 
 const WHITESPACE = /^\s+$/;