Rename `repos.yml` to `repos.json`

Github Pages now supports Jekyll 2.2, which allows for data files with
JSON extensions. We simply rename the file because its content is
already JSON-formatted; it worked before with a `.yml` extension because
YAML is a superset of JSON.
Merge pull request #5 from gds-operations/use-json-extension

Rename `repos.yml` to `repos.json`