upgrade gson to 2.3.1

Fix the StackOverFlow Exception on Throwable serialization. See changelog below:

Gson 2.3.1: Released Nov 20, 2014
- Added support to serialize objects with self-referential fields. The self-referential field is set to null in JSON. Previous version of Gson threw a StackOverflowException on encountering any self-referential fields.
  - **The most visible impact of this is that Gson can now serialize Throwable (Exception and Error)**
- Added support for @JsonAdapter annotation on enums which are user defined types
- Fixed bug in getPath() with array of objects and arrays of arrays
- Other smaller bug fixes