Travis: Use Ubuntu 16.04, use GCC 8 for Linux/GCC builds
Travis: Use Ubuntu 16.04, use GCC 8 for Linux/GCC builds

Also add one GCC 5 legacy build job for debug template.

okamstudio/godot,MarianoGnu/godot,Valentactive/godot,guilhermefelipecgs/godot,akien-mga/godot,josempans/godot,ex/godot,MarianoGnu/godot,sanikoyes/godot,firefly2442/godot,Faless/godot,akien-mga/godot,Valentactive/godot,BastiaanOlij/godot,Paulloz/godot,BastiaanOlij/godot,josempans/godot,ZuBsPaCe/godot,MarianoGnu/godot,Paulloz/godot,firefly2442/godot,firefly2442/godot,akien-mga/godot,vkbsb/godot,vnen/godot,ZuBsPaCe/godot,pkowal1982/godot,josempans/godot,honix/godot,godotengine/godot,guilhermefelipecgs/godot,Shockblast/godot,godotengine/godot,josempans/godot,honix/godot,vkbsb/godot,Zylann/godot,ZuBsPaCe/godot,BastiaanOlij/godot,ZuBsPaCe/godot,Paulloz/godot,sanikoyes/godot,vnen/godot,okamstudio/godot,Zylann/godot,Valentactive/godot,josempans/godot,pkowal1982/godot,okamstudio/godot,honix/godot,Zylann/godot,groud/godot,BastiaanOlij/godot,Faless/godot,Shockblast/godot,sanikoyes/godot,Shockblast/godot,ZuBsPaCe/godot,DmitriySalnikov/godot,BastiaanOlij/godot,firefly2442/godot,pkowal1982/godot,guilhermefelipecgs/godot,okamstudio/godot,akien-mga/godot,Faless/godot,vnen/godot,Faless/godot,MarianoGnu/godot,ex/godot,Faless/godot,DmitriySalnikov/godot,Zylann/godot,akien-mga/godot,josempans/godot,ZuBsPaCe/godot,vkbsb/godot,akien-mga/godot,godotengine/godot,sanikoyes/godot,Paulloz/godot,vkbsb/godot,Valentactive/godot,ex/godot,MarianoGnu/godot,pkowal1982/godot,pkowal1982/godot,MarianoGnu/godot,Faless/godot,Valentactive/godot,josempans/godot,okamstudio/godot,ex/godot,firefly2442/godot,okamstudio/godot,vnen/godot,Shockblast/godot,MarianoGnu/godot,pkowal1982/godot,okamstudio/godot,groud/godot,groud/godot,honix/godot,ex/godot,ex/godot,Shockblast/godot,godotengine/godot,Paulloz/godot,guilhermefelipecgs/godot,ex/godot,honix/godot,akien-mga/godot,DmitriySalnikov/godot,firefly2442/godot,Faless/godot,Shockblast/godot,okamstudio/godot,firefly2442/godot,guilhermefelipecgs/godot,godotengine/godot,vnen/godot,vkbsb/godot,sanikoyes/godot,guilhermefelipecgs/godot,godotengine/godot,sanikoyes/godot,DmitriySalnikov/godot,groud/godot,groud/godot,akien-mga/godot,Valentactive/godot,Zylann/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,Zylann/godot,vnen/godot,sanikoyes/godot,Paulloz/godot,pkowal1982/godot,godotengine/godot,vnen/godot,josempans/godot,Valentactive/godot,DmitriySalnikov/godot,DmitriySalnikov/godot,sanikoyes/godot,Zylann/godot,Valentactive/godot,BastiaanOlij/godot,godotengine/godot,Faless/godot,Shockblast/godot,honix/godot,guilhermefelipecgs/godot,okamstudio/godot,groud/godot,vkbsb/godot,DmitriySalnikov/godot,pkowal1982/godot,firefly2442/godot,vnen/godot,vkbsb/godot,Zylann/godot,Paulloz/godot,ZuBsPaCe/godot,Shockblast/godot,okamstudio/godot,BastiaanOlij/godot,vkbsb/godot,ex/godot,BastiaanOlij/godot,MarianoGnu/godot