Add versions.json endpoint with latest release information

Hello,

The attached patch adds a /versions.json endpoint with the latest release
information. The response is a machine parseable JSON array with one entry
per release.

For simplicity the patch keeps the same field names in the response as the
django model. Someone else with more familiarity with django may want to
change that (tree => major?).

Sample output:

[

{
"tree": "8.0",
"latestminor": 21,
"reldate": "2009-03-17",
"current": false,
"supported": true,
"eoldate": "2010-10-01"
},
{
"tree": "8.1",
"latestminor": 17,
"reldate": "2009-03-17",
"current": false,
"supported": true,
"eoldate": "2010-11-01"
},

// ... truncated ...

{
"tree": "8.4",
"latestminor": 0,
"reldate": "2009-07-01",
"current": false,
"supported": true,
"eoldate": "2014-07-01"
}
]

The use case is for automatically determining the latest and greatest PG
versions via simple scripting:

$ curl -s http://www.postgresql.org/versions.json | jq 'last'
{
"tree": "8.4",
"latestminor": 0,
"reldate": "2009-07-01",
"current": false,
"supported": true,
"eoldate": "2014-07-01"
}

Regards,
-- Sehrope Sarkuni
Founder & CEO | JackDB, Inc. | https://www.jackdb.com/