pass --allow-root and --config.interactive=false to bower

As mentioned in https://github.com/jakecraige/ember-cli-qunit/issues/6 I run into some problems with ember-cli-qunit, since I have to run ember-cli as root in an automation process. 

The two added flags for bower eliminate my problems. I'm not sure if it is a good solution and/or if everybody wants to have these flags enabled...