SPARK-5557: Explicitly include servlet API in dependencies.

Because of the way we shade jetty, we lose its dependency orbit
in the assembly jar, which includes the javax servlet API's. This
adds back orbit explicitly, using the version that matches
our jetty version.

Author: Patrick Wendell <patrick@databricks.com>

Closes #4411 from pwendell/servlet-api and squashes the following commits:

445f868 [Patrick Wendell] SPARK-5557: Explicitly include servlet API in dependencies.