Upgrade to Gson 2.3

This release contains the following new features:

- The new @JsonAdapter annotation to specify a Json TypeAdapter for a
  class field
- JsonPath support: JsonParser.getPath() method returns the JsonPath
  expression
- New public methods in JsonArray (similar to the java.util.List):
  contains(JsonElement), remove(JsonElement), remove(int index),
  set(int index, JsonElement element)
- Many other smaller bug fixes

See: https://groups.google.com/forum/#!topic/google-gson/MOqf5RGtIzk