chore(deps) bump lua-cassandra to 0.5.5

lua-cassandra 0.5.5 fixes a case where the cosockets were only
inserted once into the upstream connection pools. This improves
performance on setups extensively using Cassandra (such as rate-limiting
plugins).

kong-0.9.8-0.rockspec

@@ -20,7 +20,7 @@ dependencies = {
   "multipart == 0.4",
   "version == 0.2",
   "lapis == 1.5.1",
-  "lua-cassandra == 0.5.3",
+  "lua-cassandra == 0.5.5",
   "pgmoon-mashape == 2.0.1",
   "luatz == 0.3",
   "lua_system_constants == 0.1.1",