Unskip `ember-cli-head` usage test

[kratiahuja]

When #369 landed, we did not skipped the ember-cli-head addon tests in ember-cli-fastboot. This was because the addon requires to be updated per the fastboot migration guide. There is a PR in ember-cli-head when landed and released , we should skip the tests here

[melv-n]

@kratiahuja Does this cause:

FastBoot build no longer supports /Users/mirage/Projects/myproject/node_modules/ember-cli-head/app/(instance-)?initializers/browser structure. Please refer to www.ember-fastboot.com for a migration path.

I got this after upgrading from 1.0.0-beta.18 to 1.0.0-beta.19/1.0.0-rc.1.

The error message is not very helpful in that the link contains no migration support. I've reverted back to beta.18 for now.

[kratiahuja]

@mirague: please see this issue #387

[kratiahuja]

Test is unskipped.