Bump newrelic_rpm from 6.11.0.365 to 6.15.0

Bumps [newrelic_rpm](https://github.com/newrelic/newrelic-ruby-agent) from 6.11.0.365 to 6.15.0.
- [Release notes](https://github.com/newrelic/newrelic-ruby-agent/releases)
- [Changelog](https://github.com/newrelic/newrelic-ruby-agent/blob/dev/CHANGELOG.md)
- [Commits](newrelic/newrelic-ruby-agent@6.11.0.365...6.15.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Gemfile.lock

@@ -176,7 +176,7 @@ GEM
     mustermann (1.1.1)
       ruby2_keywords (~> 0.0.1)
     nenv (0.3.0)
-    newrelic_rpm (6.11.0.365)
+    newrelic_rpm (6.15.0)
     nio4r (2.5.2)
     nokogiri (1.11.2)
       mini_portile2 (~> 2.5.0)