Update debian changelog for 0.5.1.
Update debian changelog for 0.5.1.

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