travis: update docker image tag for llvm7
travis: update docker image tag for llvm7

raulgrell/zig,raulgrell/zig,zig-lang/zig,Dimenus/zig,raulgrell/zig,Dimenus/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,andrewrk/zig,Dimenus/zig