fix computed decorator syntax

[shime]

otherwise it errors out with https://i.imgur.com/lKRXjXw.png

the tests are also failing currently, seems like HTMLBars is not set up for testing properly, since it can't find Ember's default helpers (like link-to):

AppHeaderComponent: it renders
    ✘ Died on test #2     at http://localhost:7357/assets/test-support.js:2719
        at test (http://localhost:7357/assets/test-support.js:1713)
        at http://localhost:7357/assets/ember-jobs.js:3448
        at http://localhost:7357/assets/vendor.js:150
        at tryFinally (http://localhost:7357/assets/vendor.js:30)
        at http://localhost:7357/assets/vendor.js:156
        at http://localhost:7357/assets/test-loader.js:29
        at http://localhost:7357/assets/test-loader.js:21
        at http://localhost:7357/assets/test-loader.js:40
        at http://localhost:7357/assets/test-support.js:5488: Assertion Failed: HTMLBars error: Could not find component named "-link-to" (no component or template with that name was found)

[stefanpenner]

I believe the correct solution is ensuring both the ember-computed-decorators and ember-cli-babel libraries are up to date.