Travis carto update

[nebulon42]

update carto version used for Travis and target Mapnik 2.3.0 API version specifically

[yohanboniface]

Upgrading carto makes sense, but not sure why targetting 2.3.0 specifically?

[nebulon42]

Normally I would use whatever is running on the tile servers. Which is 3.0.11? But because of #2374 (comment) I thought the maintainers wanted to keep Mapnik 2 compatibility.

[yohanboniface]

ah, sure makes sense!

[pnorman]

We still have to maintain 2.1 compatibility and older Carto versions too. If we want to test more recent versions we should do it in addition.

[nebulon42]

I don't remember why, could you remind me?

[kocio-pl]

I guess this is #2198.

[nebulon42]

Ah, so it's about Tilemill again. I think this software will haunt us forever. :)

[kocio-pl]

It's rather our policy problem, since git version is already OK, but @gravitystorm doesn't feel it's enough and I don't know if TileMill team would make a release just to satisfy this requirement.

[UPDATE] it's likely.

[pnorman]

We still have to maintain 2.1 compatibility and older Carto versions too. If we want to test more recent versions we should do it in addition.