Wercker/PhantomJS keeps failing my Ember project during tests

I keep getting these errors while Worcker is performing my tests.

not ok 1 PhantomJS 2.1 - Global error: SyntaxError: Unexpected token ')' at    http://localhost:7357/assets/vendor.js, line 15740
---
    Log: |
        { type: 'error',
          text: 'SyntaxError: Unexpected token \')\' at http://localhost:7357/assets/vendor.js, line 15740\n' }
...
not ok 2 PhantomJS 2.1 - Global error: Error: Could not find module ember-metal/core required by: ember-testing/index at http://localhost:7357/assets/test-support.js, line 62
---
    Log: |
        { type: 'error',
          text: 'Error: Could not find module ember-metal/core required by: ember-testing/index at http://localhost:7357/assets/test-support.js, line 62\n' }
...
not ok 3 PhantomJS 2.1 - Global error: ReferenceError: Can't find variable: define at http://localhost:7357/assets/ocularium-frontend.js, line 9
---
    Log: |
        { type: 'error',
          text: 'ReferenceError: Can\'t find variable: define at http://localhost:7357/assets/ocularium-frontend.js, line 9\n' }
...
not ok 4 PhantomJS 2.1 - Global error: ReferenceError: Can't find variable: define at http://localhost:7357/assets/tests.js, line 3
---
    Log: |
        { type: 'error',
          text: 'ReferenceError: Can\'t find variable: define at http://localhost:7357/assets/tests.js, line 3\n' }
...
not ok 5 PhantomJS 2.1 - Global error: ReferenceError: Can't find variable: EmberENV at http://localhost:7357/5866/tests/index.html?hidepassed, line 44
---
    Log: |
        { type: 'error',
          text: 'ReferenceError: Can\'t find variable: EmberENV at http://localhost:7357/5866/tests/index.html?hidepassed, line 44\n' }
...

When I run my tests in my ember project by using, the tests pass.

ember test

It's only in Wercker and it's usage of PhantomJS that the tests fail. I assume this has something to do that PhantomJS doesn't support ES6? But in my .jshintrc I added this line:

"esversion": 6,

Is there a way to let these tests pass?

---

These are my steps in wercker.yml:

steps:
- script:
  name: yarn-install
  code: npm install -g yarn

- script:
  name: bower-install
  code: yarn global add bower

- script:
  name: phantomjs-prebuilt-install
  code: yarn global add phantomjs-prebuilt

- script:
  name: ember-cli
  code: yarn global add ember-cli

# A step that executes `npm install` command
- script:
  name: install-deps
  code: yarn install

- script:
  name: bower-install-deps
  code: bower install --allow-root

- script:
  name: tests
  code: ember test

- script:
  name: ember-build
  code: ember build