add github json data for mbta-alerts fixture

This isn't *exactly* the same as the other githubDetails attributes for the other fixtures. using [this json](https://apigee.com/console/github?req=%7B%22resource%22%3A%22getrepo%22%2C%22params%22%3A%7B%22query%22%3A%7B%7D%2C%22template%22%3A%7B%22userName%22%3A%22codeforboston%22%2C%22repoName%22%3A%22mbta-alerts%22%7D%2C%22headers%22%3A%7B%7D%2C%22body%22%3A%7B%22attachmentFormat%22%3A%22mime%22%2C%22attachmentContentDisposition%22%3A%22form-data%22%7D%7D%2C%22verb%22%3A%22get%22%7D) we do not get some permissions object which is present in other fixtures. Guessing this is a fragment from the past API where the initial fixtures were generated.