Stop specifying requirements.txt in tox.ini

As PBR already injects the contents of requirements.txt into
setup_requires, there is no need for tox to explicitly install them.
http://lists.openstack.org/pipermail/openstack-dev/2015-July/069663.html

Change-Id: I5fc4de208e6c2f6926238f6942bf4ca9804748e4

tox.ini

@@ -7,8 +7,7 @@ envlist = pep8, py27, pypy, py33, py34
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
 install_command = pip install {opts} {packages}
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
 commands = flake8