requirements.txt: pin down the version of sphinxcontrib-httpdomain
requirements.txt: pin down the version of sphinxcontrib-httpdomain

Pin down the version of sphinxcontrib-httpdomain to 1.3.0 as more
recent versions are not compatible with Sphinx 1.4.9 which is already
pinned down.

Signed-off-by: Guillaume Tucker <e7e20b7c30f19d4ca6b81319ef81bf200369d137@collabora.com>

kernelci/kernelci-backend,kernelci/kernelci-backend