If the Travis failure was just temporary you should be able to restart the Travis run for the tag and it should deploy.
If the documentation does not build on that tag however, then you have to build it manually and push it to gh-pages. If you use the same naming convention as Documenters deployed docs it should be picked up in the version selector during the next real Documenter run. It can sometimes be difficult to build the correct docs locally, e.g. links to the repository etc may not work.
I would argue that the documentation for old releases are not that important anyway, and you might just as well fix your issues and make a new release that has working docs to avoid this hassle.
Edit: It is a good idea to check that the documentation builds correctly on
master, just as you are checking that the tests pass, before tagging a new release.