remove --pre option from tox's pip install command

By default, tox passes --pre option to pip install commands so that
prerelease packages, which perhaps are not suitable for test,
would be installed, causing unforseeable errors. Like:
http://logs.openstack.org/33/47233/1/check/gate-zuul-docs/7b794af/console.html
whose installed Sphinx package is a beta version 2 causing the error in the log.
fungi's proposal https://review.openstack.org/#/c/47239/1 also applies here.

Change-Id: I9108b534bb469211434a4abf22b25c983aa444ba