Revert Jetty Maven plugin bump

Jetty 9.4.20 has a breaking change causing issues with Spring's
Websocket handling. This will be fixed in version 9.4.21.

See https://github.com/eclipse/jetty.project/issues/4020.

Closes #49.

pom.xml

@@ -363,7 +363,7 @@
 			<plugin>
 				<groupId>org.eclipse.jetty</groupId>
 				<artifactId>jetty-maven-plugin</artifactId>
-				<version>9.4.20.v20190813</version>
+				<version>9.4.19.v20190610</version>
 				<configuration>
 					<scanIntervalSeconds>0</scanIntervalSeconds>
 					<webApp>