Bump newrelic_rpm from 6.3.0.355 to 6.4.0.356

Bumps [newrelic_rpm](https://github.com/newrelic/rpm) from 6.3.0.355 to 6.4.0.356.
- [Release notes](https://github.com/newrelic/rpm/releases)
- [Changelog](https://github.com/newrelic/rpm/blob/master/CHANGELOG.md)
- [Commits](newrelic/newrelic-ruby-agent@6.3.0.355...6.4.0.356)

Gemfile.lock

@@ -166,7 +166,7 @@ GEM
     minitest (5.11.3)
     mustermann (1.0.3)
     nenv (0.3.0)
-    newrelic_rpm (6.3.0.355)
+    newrelic_rpm (6.4.0.356)
     nio4r (2.3.1)
     nokogiri (1.10.3)
       mini_portile2 (~> 2.4.0)