installation: mercurial addition
installation: mercurial addition

* Adds mercurial to the list of dependencies for installation
  in INSTALL guide after 787b62e8.

Signed-off-by: Jan Aage Lavik <8d55e006a1b29770f275a5c59a7341f17f8e69d5@cern.ch>

tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio