Upgrade New Relic

Previously, the demo used the New Relic Java Agent version 2.21.0.
Based on the documentation in https://newrelic.com/docs/releases/java,
this had a bug that caused performance issues on startup.  This change
updates the version of New Relic to 2.21.3 which solves the
performance problem.
Merge pull request #2 from nebhale/upgrade-new-relic

Upgrade New Relic