Bug in custom tab bar

https://github.com/codepath/ios_guides/wiki/Creating-a-Custom-Tab-Bar

After following the tutorial, I successfully run the custom tab bar app. However, there will be a blank gap between tab bar view and the first content view loaded in `viewDidLoad()`. Other content view works great.

Update: My content view is with scroll view, and it's the bug origin.

Start with the Creating a Custom Tab Bar tutorial linked in the issue and reproduce the blank gap using a content view whose layout includes a scroll view. Compare that result with the other content views and identify the layout state created during viewDidLoad(). Done means the scroll-view content no longer leaves a gap below the tab bar while the other content views continue to work.

Written by the indexing model from the issue text.