change day 9-12 requirements.txt and readme.md Day3 instruction to apistar==0.5.41 for package 0.6+ breaking changes

[ericchou1]

Raised a potential issue #5 for apistar package changes from 0.5.41 to 0.6+ that did away with servers (http://docs.apistar.com/). Not sure if my interpretation is correct, at least 0.7.2 did not work on my setup while 0.5.41 works, submitting PR in case this was indeed the case.

[bbelderbos]

requirements.txt had the fix but forgot to document it in the README so thanks for that change @ericchou1, and for merging it in @mikeckennedy. I am adding a callout in the video where I pip install apistar as well.

[mikeckennedy]

Thanks Bob. I just published the updated videos. Will take up to 5 min to replicate to your region. :)