Install requirements.txt in tox venvs as well

This allows things like tox -e venv to work as well, so you can do

    tox -e venv -- python
    >>> from cratonclient ... import ...

Without getting an ImportError for a missing dependency.

Change-Id: I75955c6fe7d6b4403eff8995932ff3ea3cb961b3

tox.ini

@@ -8,7 +8,9 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+   -r{toxinidir}/requirements.txt
+   -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]