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

从构建仓库 JSON 响应的 `/` 端点开始，跟踪仓库数据的获取和缓存方式。将每个仓库最新的 GitHub release 标签用作 `release` 值，然后验证每个返回的仓库都在文档所述的响应结构中包含该字段。

由索引模型根据 Issue 内容生成。