Fix tox's insane pip install command.

Override tox's pip install command so that prereleases (alphas, betas,
broken things) aren't installed by default. Instead only install release
versions unless a prerelease version is specifically provided. Do this
by overriding the tox install command value.

This fixes doc builds that use `tox -evenv python setup.py
build_sphinx`.

Also, remove the gearman servers from the test configuration (since there
is not yet any support for it).

Change-Id: I31913420adcb48866d3996f2dd3b605c55acce2e