Update newrelic plugin version.

Changelog: https://docs.newrelic.com/docs/release-notes/agent-release-notes/python-release-notes

requirements/edx/base.txt

@@ -99,7 +99,7 @@ watchdog==0.7.1
 
 # Metrics gathering and monitoring
 dogapi==1.2.1
-newrelic==2.46.0.37
+newrelic==2.56.0.42
 
 # Used for documentation gathering
 sphinx==1.1.3