Fixed Travis to exit early on lint failures and skip duplicate linting in ember tests

no issue
- Travis runs all `script` tasks even if an earlier one fails so moved to using `&&` so ember tests aren't run if linting errors
- Added `ember-cli-eslint` to the addons blacklist when building in Travis so that we aren't running linting twice (also reduces build log output)

ghost/admin/.travis.yml

@@ -31,8 +31,7 @@ install:
   - yarn
 
 script:
-  - npm run lint:js
-  - COVERAGE=true npm test
+  - npm run lint:js && COVERAGE=true npm test
 
 after_success:
   - npm run coverage

ghost/admin/ember-cli-build.js

@@ -105,8 +105,14 @@ const simplemdeAssets = function () {
     return config;
 };
 
+let blacklist = [];
+if (process.env.CI) {
+    blacklist.push('ember-cli-eslint');
+}
+
 module.exports = function (defaults) {
     let app = new EmberApp(defaults, {
+        addons: {blacklist},
         'ember-cli-babel': {
             optional: ['es6.spec.symbols'],
             includePolyfill: false