Fix travis build

Signed-off-by: Dan O'Reilly <[email protected]>
diff --git a/python/tox.ini b/python/tox.ini
index ebe7545..d758512 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -6,6 +6,7 @@
     py{26,27}-{cpp,python}
 
 [testenv]
+usedevelop=true
 setenv =
     cpp: LD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
@@ -16,6 +17,7 @@
     python: python setup.py -q test -q
     cpp: python setup.py -q test -q --cpp_implementation
 deps =
+    # Keep this list of dependencies in sync with setup.py.
     six
     py26: ordereddict
     py26: unittest2