Add six as explicit dep of boto_test (#131)

This fixes the breakage in #98 by working around #70.

examples/boto/BUILD

diff --git a/examples/boto/BUILD b/examples/boto/BUILD
index 0f22871..414f6df 100644
--- a/examples/boto/BUILD
+++ b/examples/boto/BUILD
@@ -21,5 +21,10 @@
 py_test(
     name = "boto_test",
     srcs = ["boto_test.py"],
-    deps = [requirement("boto3")],
+    deps = [
+        requirement("boto3"),
+        # six is a transitive dependency via python-dateutil. Explicitly depend
+        # on it to work around issue #70; see issue #98.
+        requirement("six"),
+    ],
 )