Bump jetty-maven-plugin from 9.3.9.v20160517 to 9.4.15.v20190215

Bumps [jetty-maven-plugin](https://github.com/eclipse/jetty.project) from 9.3.9.v20160517 to 9.4.15.v20190215.
- [Release notes](https://github.com/eclipse/jetty.project/releases)
- [Commits](jetty/jetty.project@jetty-9.3.9.v20160517...jetty-9.4.15.v20190215)

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

Author: dependabot-support

chat-example/chat-vaadin-client/pom.xml

@@ -22,7 +22,7 @@
     <maven.compiler.source>1.8</maven.compiler.source>
     <maven.compiler.target>1.8</maven.compiler.target>
 
-    <jetty.plugin.version>9.3.9.v20160517</jetty.plugin.version>
+    <jetty.plugin.version>9.4.15.v20190215</jetty.plugin.version>
     <vaadin.version>8.1.6</vaadin.version>
     <!-- If there are no local customisations, this can also be "fetch" or "cdn" -->
     <vaadin.widgetset.mode>cdn</vaadin.widgetset.mode>