Add .tox to .gitignore

Change-Id: I6c6be72557bd10e2e93b7bd8da6601b198419dca
diff --git a/.gitignore b/.gitignore
index 89962f0..e5106f0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,3 +9,4 @@
 *.log
 *.swp
 *.egg-info
+.tox