Fix iOS 11.2 regression when tapping to jump

[TimOliver]

iOS 11.2 (or possibly earlier) has introduced a regression where tapping down on the track no longer instantly jumps to that section of the scroll view.

Current behavior is that the jump will occur after the user has lifted their finger, or will merely happen on its own after a second if the finger is left down.

This 'delayed' activity seems to imply it's the result of an internal gesture recognizer taking too long to release its control.

Since UIScrollView uses gesture recognizers internally, I'm GUESSING the best way to fix this is to wrap all of the scroll bar's interactions into a custom recognizer that can then be used to override the scroll view.

[TimOliver]

Fixed in PR #8.