Rename link to url in manup json

[MichaelMarner]

link is kind of a weird word. Suggest renaming to url, as it is a more standard way of referring to a, well, url.

Manup metadata would then look like this:

{
  "ios": {
    "minimum": "1.0.2",
    "latest": "2.1.0",
    "url": "http://example.com",
    "enabled": true
  }
}