Important fix for DUB snap package

[Joseph Rushton Wakeling]

Hello all,

I've just released an important fix for the DUB snap package: it 
now bundles its own libcurl.

This should prevent issues observed on Ubuntu 18.04 where the dub 
snap was unable to find a suitable libcurl to load, and therefore 
could not download package data.

To upgrade to this new version, use:

     sudo snap refresh --classic --channel=stable dub

This release also updates DUB itself to v1.13.0.

Thanks to Phil Burr who submitted a PR with the fix, and 
apologies to anyone who was impacted by the problem.

Best wishes,

     -- Joe