travis.yml for MacOS tried to update documentation

The travis.yml executes ./scripts/update-docs.sh.
I think that this should have a guard such that it runs only when within the primary tinycbor repository and/or it should be followed by || exit 0, as I don't think failure to update the docs should be a cause for the regression tests to fail.

It also seems to fail for macOS build only. Maybe because the listed code sizes changes on macOS only, so that's the only case where the docs need to be updated.

I can send a PR, if you advise how you think you want this fixed.