Error fetching if repo contains no tags

[Carl Fürstenberg]

If a repo contains no tags, then git fetch --depth=1 shipit --prune "refs/tags/*:refs/tags/*"' will return 1 and deployment halts.

[Tim Kelty]

Good catch. Open to a PR, otherwise I should be able to take a look soon.

[Greg Bergé]

shipitjs/shipit#167