[SPARK-959] Explicitly depend on org.eclipse.jetty.orbit jar

Without this, in some cases, Ivy attempts to download the wrong file
and fails, stopping the whole build. See (bug)[https://spark-project.atlassian.net/browse/SPARK-959] for more details.

Note that this may not be the best solution, as I do not understand the root cause of why this only happens for some people. However, it is reported to work.,