Travis: Use Xcode 11.3 for macOS/iOS
Travis: Use Xcode 11.3 for macOS/iOS

Xcode 10+ is needed for exhaustive C++17 support (gnu++17).

11.3 is the latest available version on Travis, and we don't have a
specific reason not to use it.

Follow-up to #36457.

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