travis: start testing both mapnik v2.2.0 and v2.3.x
travis: start testing both mapnik v2.2.0 and v2.3.x

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