Make 'pushed_at' an optional field.

Some repos appear to have it unset, e.g.
https://github.com/djs55/scripts

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Update CHANGES

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #27 from jonludlam/updates2

Make 'pushed_at' an optional field.