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: If624cea7246e0a16355923af122b2f8586718d5e

tox.ini

@@ -28,9 +28,6 @@ setenv = PYTHON=coverage run --source diskimage_builder
 commands = bash -c 'if [ ! -d ./.testrepository ] ; then testr init ; fi'
            bash -c 'testr run --parallel ; RET=$? ; coverage combine ; coverage html -d ./cover $OMIT && exit $RET'
 
-[tox:jenkins]
-downloadcache = ~/cache/pip
-
 [testenv:docs]
 commands = python setup.py build_sphinx