Update INSTALL.md
Update INSTALL.md

listed required PHP extensions.
stopfstedt/ilios,Trott/ilios,ilios/ilios,justinchu17/ilios,ilios/ilios,Trott/ilios,dartajax/ilios,justinchu17/ilios,thecoolestguy/ilios,thecoolestguy/ilios,justinchu17/ilios,justinchu17/ilios,dartajax/ilios,stopfstedt/ilios