HTTPretty: bump to 0.8.0

This new version brings Python 3 support, leaner requirements and better
packaging. Python-keystoneclient needs this to be Python 3 compatible.

Change-Id: I85298e7b99cfb1abec42058e63ee419e46900cb4
Allow webob 1.3

Both Trusty and Sid have WebOb 1.3, so we need to allow
WebOb 1.3.

Change-Id: I1c64cf4ff659b6a2ef0d6915bf83f9eb5e44a8e9
Merge "HTTPretty: bump to 0.8.0"