chore(deps) bump resty.dns.client from 5.1.0 to 5.1.1

### Summary

Only bumps its own dependency on `lua-resty-timer`. This has become
an issue with `lua-resty-healtchecks` bump that also has `lua-resty-timer`
dependency now, but for different version. LuaRocks loader has been found
flaky on a platform when there are more than two versions of the same library
installed. Luarocks also does not seem to handle downgrades well, or cleanups
with updates in our use case

kong-2.2.0-0.rockspec

@@ -28,7 +28,7 @@ dependencies = {
   "lyaml == 6.2.5",
   "luasyslog == 1.0.0",
   "lua_pack == 1.0.5",
-  "lua-resty-dns-client == 5.1.0",
+  "lua-resty-dns-client == 5.1.1",
   "lua-resty-worker-events == 2.0.0",
   "lua-resty-healthcheck == 2.0.0",
   "lua-resty-cookie == 0.1.0",