Bump jetty-server in /rejoiner-schema

Bumps [jetty-server](https://github.com/eclipse/jetty.project) from 9.4.8.v20171121 to 9.4.17.v20190418.
- [Release notes](https://github.com/eclipse/jetty.project/releases)
- [Commits](jetty/jetty.project@jetty-9.4.8.v20171121...jetty-9.4.17.v20190418)

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

rejoiner-schema/pom.xml

@@ -26,7 +26,7 @@
     <dependency>
       <groupId>org.eclipse.jetty</groupId>
       <artifactId>jetty-server</artifactId>
-      <version>9.4.8.v20171121</version>
+      <version>9.4.17.v20190418</version>
     </dependency>
     <dependency>
       <groupId>com.google.api.graphql</groupId>