INSTALL.md: fix note about postgis 2.1.3
INSTALL.md: fix note about postgis 2.1.3

shadowoneau/skylines,TobiasLohner/SkyLines,shadowoneau/skylines,skylines-project/skylines,shadowoneau/skylines,Harry-R/skylines,Turbo87/skylines,skylines-project/skylines,snip/skylines,Turbo87/skylines,Harry-R/skylines,Turbo87/skylines,kerel-fs/skylines,shadowoneau/skylines,Harry-R/skylines,TobiasLohner/SkyLines,TobiasLohner/SkyLines,kerel-fs/skylines,snip/skylines,snip/skylines,RBE-Avionik/skylines,kerel-fs/skylines,skylines-project/skylines,RBE-Avionik/skylines,Turbo87/skylines,skylines-project/skylines,RBE-Avionik/skylines,Harry-R/skylines,RBE-Avionik/skylines