Deprecated tox -downloadcache option removed

Caching is enabled by default from pip version 6.0

More info:
https://testrun.org/tox/latest/config.html#confval-downloadcache=path
https://pip.pypa.io/en/stable/reference/pip_install/#caching

Change-Id: I4d0b1432e5e8e60bca06fa094a99946523155af6

tox.ini

@@ -15,9 +15,6 @@ deps = -r{toxinidir}/requirements.txt
 commands =
   python setup.py testr --testr-args='{posargs}'
 
-[tox:jenkins]
-downloadcache = ~/cache/pip
-
 [testenv:pep8]
 commands = flake8
 
@@ -33,4 +30,4 @@ commands=
 
 [flake8]
 show-source = True
-exclude = .venv,.git,.tox,dist,*lib/python*,*egg,build
+exclude = .venv,.git,.tox,dist,*lib/python*,*egg,build