Fixes usage of velocity tracker due to breaking change in Flutter

[josh-burton]

Fixes #193.

[kleeb]

+1 for merging

[KieranLafferty]

@akshathjain is this repo still being maintained? Noticed that there haven't been any changes in the last 5 months just wondering if it's likely these PRs will get merged in

[omidraha]

Please merge.

[pcl]

FYI you can depend on @josh-burton's version like so:

dependencies:
  sliding_up_panel:
    git:
      url: git@github.com:josh-burton/sliding_up_panel
      ref: 52e53f93b14dd2db4a8f758ddf30d319be69c884

I recommend using the SHA for the ref, and not the branch, since the SHA also serves to validate that the contents are the same as you expect them to be.

[sidrao2006]

This solution was obsoleted by another flutter update - https://api.flutter.dev/flutter/gestures/VelocityTracker/VelocityTracker.html