add version number for each repo

Currently only the `bigchaindb/bigchaindb` repo gets a version number in its json response.

### Scope

Include version number based on the latest GitHub release tag for each repo as value of a `release` key in the `/` endpoint. Like so:

```json
[
{
"name": "project-name",
"description": "The description",
"stars": 3040,
"forks": 293,
"is_fork": false,
"release": "v0.10.0",
"url": "https://github.com/bigchaindb/project"
}
]
```

Needed for solving https://github.com/ascribe/bigchain-website/issues/139

Start at the `/` endpoint that builds the repository JSON response and trace how repository data is fetched and cached. Use the latest GitHub release tag for each repository as the `release` value, then verify that every returned repository includes the field in the documented response shape.

Vom Indexierungsmodell aus dem Issue-Text verfasst.