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: I481d8a9e1f079dcf8e135d18a665cc5df038b803
diff --git a/tox.ini b/tox.ini
index fd5d24e..cfd9e06 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,9 +19,6 @@
 commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
-[tox:jenkins]
-downloadcache = ~/cache/pip
-
 [testenv:pep8]
 commands = flake8