Make gson a provided dependency

gson is contained in Gerrit core and the plugin doesn't need to bring
it once more.

Change-Id: Ibef784abd9e86773eb98b536de26f955130ffa32
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/BUCK b/BUCK
index 4a60691..b3aae10 100644
--- a/BUCK
+++ b/BUCK
@@ -26,10 +26,10 @@
   ],
   deps = [
     HTTP_LIB,
-    GSON,
   ],
   provided_deps = [
     LOG4J,
+    GSON,
   ],
 )