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: I886027eb07ed92da7564103a27c962c0fb8d1e08

tox.ini

@@ -13,9 +13,6 @@ deps =
     -r{toxinidir}/test-requirements.txt
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
-[tox:jenkins]
-downloadcache = ~/cache/pip
-
 [testenv:bashate]
 whitelist_externals = bash
 commands = bash -c "find {toxinidir} -not -wholename \*.tox/\* -and \( -name \*.sh -or -wholename \*.d/\* -and -not -name \*.md -and -not -name \*.rst -and -not -name \*.py \) -print0 | xargs -0 bashate -v"