add version number for each repo

[kremalicious]

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:

[
  {
    "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 bigchaindb/site#139