tox: make pip install process dependency_links

needed as long as we need the git version of python-blosc, should be fixed soon by python-blosc 1.2.5.

tox.ini

@@ -4,6 +4,7 @@ envlist = py32, py33, py34
 [testenv]
 # Change dir to avoid import problem
 changedir = docs
+install_command = pip install --process-dependency-links {opts} {packages}
 commands = {envpython} -m attic.testsuite.run -bv []
 
 [testenv:py32]