Don't use/poison global wheel cache when building w/ PURE_PYTHON.

CHANGES.rst

@@ -4,6 +4,9 @@ Changes
 4.2.0 (unreleased)
 ------------------
 
+- When testing ``PURE_PYTHON`` environments under ``tox``, avoid poisoning
+  the user's global wheel cache.
+
 - Drop support for Python 2.6 and 3.2.
 
 - Add support for Python 3.5.

tox.ini

@@ -7,19 +7,21 @@ envlist =
 
 [testenv]
 commands = 
-    python setup.py test -q
+    python setup.py -q test -q
 
 [testenv:py27-pure]
 basepython =
     python2.7
 setenv =
     PURE_PYTHON = 1
+    PIP_CACHE_DIR = {envdir}/.cache
 
 [testenv:py33-pure]
 basepython =
     python3.3
 setenv =
     PURE_PYTHON = 1
+    PIP_CACHE_DIR = {envdir}/.cache
 
 [testenv:jython]
 commands =