installation: fix for jQuery hotkeys add-on URL

* Uses stable version of jQuery hotkeys plugin from
  invenio-software.org.  (closes #1507)

Reported-by: Theodoropoulos Theodoros <theod@lib.auth.gr>
Co-authored-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>