sync the osx and ubuntu installation instructions by providing download link for ubuntu - closes issue #420 - thx @nvkelso for catch
sync the osx and ubuntu installation instructions by providing download link for ubuntu - closes issue #420 - thx @nvkelso for catch

mbrukman/tilemill,tilemill-project/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,florianf/tileoven,isaacs/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,paulovieira/tilemill-clima,tizzybec/tilemill,florianf/tileoven,mbrukman/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,nyimbi/tilemill,mbrukman/tilemill,isaacs/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,tizzybec/tilemill,MappingKat/tilemill,tilemill-project/tilemill,nyimbi/tilemill,tizzybec/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,paulovieira/tilemill-clima,nyimbi/tilemill,MappingKat/tilemill,nyimbi/tilemill,MappingKat/tilemill,tizzybec/tilemill,MappingKat/tilemill,nyimbi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,isaacs/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,florianf/tileoven,MappingKat/tilemill,mbrukman/tilemill,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,mbrukman/tilemill,Zhao-Qi/tilemill,tilemill-project/tilemill,fxtentacle/tilemill,tizzybec/tilemill