Install lets encrypt for vmap0.tiles.osgeo.org

[robe]

​https://lists.osgeo.org/pipermail/sac/2019-May/011079.html

> I am submitting a couple patches to the Cartopy project to ensure HTTPS is
> used when downloading resources.
>
> While working on these patches, I noticed that there appears to be an
> issue with the certificate served by https://vmap0.tiles.osgeo.org -- the
> certificate is only valid for https://webextra.osgeo.osuosl.org. As a
> result, modern browsers and HTTP client libraries will perform the "safer"
> behavior of failing. Below is an example
> from the Python Requests library exhibiting this behavior.
>
> Best regards,
> Jon Szymaniak
>
> ----
>

[robe]

Should be fixed now.