Added efficiency improvement to get_installation

[lrjball]

I've been using this project as a template for my own github app and it has been really helpful, so thank you.

When looking through I did notice one slight enhancement - github has an endpoint to get the installation directly given a user which is easier than getting every installation of the app and looping through them all to find the user.

Hopefully this is helpful, I have tested it on my own app and the two functions return exactly the same data. I couldn't think of an easy way to add a test for this but if anyone can suggest one then I can add it in.

[python-cla-bot[bot]]

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