Update jetty-maven-plugin.xml

In the declaration of the org.eclipse.jetty plugin is indicated the ${project.version} property, but should be indicated the plugin version.
This property should be defined in the properties section of the pom file.

<properties>
  <jettyVersion></jettyVersion>
</properties>
Merge pull request #22 from ghost/patch-1

Update jetty-maven-plugin.xml reference to jetty version