This change also fixed a problem we had with ckanext-harvest. html5lib-0.999999999 was demanding a newer version of setuptools than travis gave it by default:
html5lib requires setuptools version 18.5 or above; please upgrade before installing (you have 12.0.5)
But hacking the build script to use ckan branch release-2.5.3, it gets bleach 1.4.3 and now pulls in html5lib-0.9999999 which doesn't have this error.
I won't merge the ckanext-harvest hack though, but rest assured it'll go green when the ckan release is done.