Pass Slick version properly from sbt into Sphinx.

sbt-site automatically sets 'release' and 'version' properties for
Sphinx but these are only visible in .rst and template HTML files
whereas conf.py uses its own version and release defined there. This
required updating the version numbers in conf.py for every release
and, due to the way the short 'version' is computed from the proper
'release' by sbt-site, lead to issues such as the Slick 2.0.0 docs
showing only '2.0' instead of '2.0.0' in the title.

Upgrading to sbt-site 0.7.1, disabling the automatic version properties
passing, and passing them manually through env variables into conf.py
should fix this once and for all.

Review by @cvogt