Add gelk version to panel JSON exported by Kidash

[alpgarcia]

It would be very useful to store Gelk version together with panels. This way we will know whether data in an index is fully compatible with that panel.

Kidash could export highest version found in the index where the panel is exported from and store it as a new field in panel's JSON file. As one panel can deal with several indices, the object could be something like:

'gelk_version': {
    'git': 'x.x.x',
    'gerrit': 'x.x.x',
    ...
}

This feature outcome will allow to develop https://github.com/grimoirelab/panels/issues/108