Upgrade gson to 2.2.4 due to hashing bug in 2.2.3

I uncovered an intermittent and very hard to detect hashing bug in gson 2.2.3.  According to gson roadmap (see here: https://sites.google.com/site/gson/gson-roadmap), this bug was fixed in 2.2.4.  I was able to confirm this in my project by excluding 2.2.3 from jest and including 2.2.4 directly into my project.  Let me know if you have any questions.