Don't use a beginning slash with s3 datasources.
Don't use a beginning slash with s3 datasources.

If we do general-purpose S3, we'll need to redo this,
but it's good enough for the time being. Fixes #959

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