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: I9728aabe59d6e8edd8496e3126bac2057755fee2

tox.ini

@@ -7,9 +7,6 @@
 skipsdist = True
 envlist = pep8
 
-[tox:jenkins]
-downloadcache = ~/cache/pip
-
 [testenv:pep8]
 # TODO: #deps = hacking==0.7
 deps = flake8
@@ -22,4 +19,4 @@ ignore = H302,H802
 exclude = .venv,.git,.tox,dist,doc,*egg,build
 show-pep8 = True
 show-source = True
-count = True
+count = True