Support ember-cli-fastboot rc1

[mschinis]

Fastboot rc1 released three days ago made a couple of breaking changes, one of them is using (instance-)initialisers/browser structure.

Ember serve fails with:

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

We should either:

1. do a quick temporary solution to set ember-cli-fastboot dependency to be exact 1.0.0-beta.16 (last known working beta version with this project)
2. Update ember-cli-head and ember-api-docs to not use these initializers.

Option number 2 is depends on ember-fastboot/ember-cli-head#21