Store data about found/checked repositores in JSON format

[gabrielskoglund]

We currently store the found/checked git repositores in the dataset/found_repositories and dataset/checked_repositories files. In order to improve data gathering, we should instead store this data in JSON format containing the following:

• The name of the repository
• The date on which the repository was scanned. This would allow us to only check newer PRs when we look for new breaking updates for this repo.

By using this format, all data could be stored in a single JSON document, since a repo with no date of scan is unchecked.

In order to resolve this issue, both Java code and existing data needs to be updated.

[gabrielskoglund]

This issue has been resolved by #20