Travis-CI: Change lowest Sphinx version from 1.8.0 to 1.8.1
Travis-CI: Change lowest Sphinx version from 1.8.0 to 1.8.1

This caused an error when building LaTeX and epub.
Probably not worth investigating further ...

spatialaudio/nbsphinx,spatialaudio/nbsphinx,spatialaudio/nbsphinx