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

tox.ini

@@ -1,4 +1,5 @@
 [tox]
+minversion = 1.6
 envlist = pep8, pyflakes, py27
 
 [testenv]
@@ -6,6 +7,7 @@ envlist = pep8, pyflakes, py27
 setenv = STATSD_HOST=localhost
          STATSD_PORT=8125
          VIRTUAL_ENV={envdir}
+install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =