README.md: pipenv.gif 404's

[jasonharrison]

In README.md

![image](https://s3.amazonaws.com/media.kennethreitz.com/pipenv.gif)

https://s3.amazonaws.com/media.kennethreitz.com/pipenv.gif seems to 404.

db3b743#diff-04c6e90faac2675aa89e2176d2eec7d8R23

[frostming]

Can you see the GIF on Github repo page?

[jasonharrison]

No. That's how I noticed the problem.

[frostming]

We have replaced the GIF on the doc page, would you like to fix it for README? PR welcome.

[jasonharrison]

@frostming Sure! PR is here: #3912