Add installation field to PushEvent

[jkarni]

Issue reference: #91

Submission Checklist:

• [x] Have you followed the guidelines in our Contributing document (for example, is your tree a clean merge)?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Does your submission build?
• [x] Does your submission pass tests?
• [x] Have you run lints on your code locally prior to submission?
• [x] Have you updated all of the cabal/nix infrastructure?
• [x] Is this a breaking change? Have you discussed this? [Not breaking unless any new field is]

[ashercuedo]

Thanks for putting in the pull request, I'll hop on it as soon as I can 👍