Add dependency of gson

Else buck build will run into error.

Change-Id: I1a69c4a74478475d85f4f96710a7f594da5e156d

BUCK

diff --git a/BUCK b/BUCK
index f5407b7..4eb7273 100644
--- a/BUCK
+++ b/BUCK
@@ -9,6 +9,7 @@
   ],
   deps = [
     '//lib/commons:io',
+    '//lib:gson',
   ],
 )