Make sure tox does not call pip install with --pre

* tox.ini: By default tox passes --pre to pip install commands so
that prerelease packages will be deemed automatically suitable, as a
means of keeping backwards-compatible with earlier pip releases
which did not interpret version strings looking for potential
prerelease patterns. By overriding the install_command in tox 1.6
and later, this option can be removed from the pip command line so
that prerelease versions of packages will only be installed if a
version spec for a particular package is also a prerelease.

Change-Id: I978b906b0ecd6c01c99e08f9d27271a1f31c7a14

tox.ini

@@ -1,4 +1,5 @@
 [tox]
+minversion = 1.6
 envlist = pep8, pyflakes, py26, py27
 
 [tox:jenkins]
@@ -8,6 +9,7 @@ downloadcache = ~/cache/pip
 setenv VIRTUAL_ENV={envdir}
        SUBUNIT_FORMATTER=tee testr_subunit_log
        OS_STDOUT_NOCAPTURE=False
+install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/tools/pip-requires
        -r{toxinidir}/tools/test-requires
 commands = python setup.py testr --slowest --testr-args='{posargs}'