v1.3 documentation is now active

[dsetrakyan]

Igniters,

I went ahead and made 1.3 an active version of Ignite documentation:
http://apacheignite.readme.io/docs

Unfortunately, in readme.io updates to the previous version do not
automatically propagate to the next version, so it is very painful to
maintain several active versions.

Let's always switch to the latest (even though it may not has been released
just yet).

D.