Hotfix 4.6.1
Travis is currently only deploying egg files to pypi. Fix this by forcing the sdist to be built before deploy.
This hotfix only affects CI (i.e. for a user v4.6.1 is completely equivalent to v4.6.0)
Note: this is a second attempt to hotfix the same. The previous one was #989 which was applied but which will be overwritten (after a forced-push to revert)