Remove minimal-ui viewport

The `minimal-ui` viewport property was introduced in iOS 7.1, but will no longer be supported in iOS 8 [http://bit.ly/1pjQ8xe](http://bit.ly/1pjQ8xe)

It is recently removed from the html5-boilerplate repo, so I think we should not use it anymore in the mobile-boilerplate too. [https://github.com/h5bp/html5-boilerplate/pull/1565](https://github.com/h5bp/html5-boilerplate/pull/1565)