Downstream updates from stapler/stapler#131

The stapler API was built using servlet 2.5 and yet Jenkins uses 3.1.
This meant some of the code that stapler used was missing new methods
that where part of the updated spec and if a plugin happened to call
them you would end up with a LinkageError or some other crazyness.

This also make Jenkins depend on a dummy version of the old serlet-api
maven co-ordinates such that if any plugin gets aa dependency on it
transitivly (e.g. Jenkins test harness) they will get a version with no
code so the classpath should always be clean (this was more an issue for
eclipse than an mvn command)