Add google.auth.crypt (#6)

diff --git a/tox.ini b/tox.ini
index d2a5f1f..5f2df1c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -25,7 +25,9 @@
 deps =
   {[testenv]deps}
   sphinx
-commands = sphinx-apidoc --output-dir docs/reference --separate --module-first google
+commands =
+  rm -r docs/reference
+  sphinx-apidoc --output-dir docs/reference --separate --module-first google
 
 [testenv:docs]
 basepython = python3.5