jsduck: Set --processes=0 to fix warnings-exit-nonzero
jsduck: Set --processes=0 to fix warnings-exit-nonzero

Workaround for https://github.com/senchalabs/jsduck/issues/525.

This was previously overridden by Jenkins, but the repo is now
expected to have its own configuration. There was some confusion
about it applying to Ruby 1.8 / Ubuntu Precise only, but even on
Trusty / Ruby 1.9 the multi-process is unstable.

Change-Id: Ic3083e39c22c9f545e3f7f1ebd9356fb23d08ad1

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui