Remove double-"brew update" as now unnecessary
Remove double-"brew update" as now unnecessary

The issue is closed and our latest runs on Travis are only needing to
run brew update once.

louiscryan/grpc-java,carl-mastrangelo/grpc-java,carl-mastrangelo/grpc-java,rmichela/grpc-java,zpencer/grpc-java,anuraaga/grpc-java,grpc/grpc-java,anuraaga/grpc-java,carl-mastrangelo/grpc-java,madongfly/grpc-java,stanley-cheung/grpc-java,pieterjanpintens/grpc-java,zhangkun83/grpc-java,pieterjanpintens/grpc-java,dapengzhang0/grpc-java,eonezhang/grpc-java,louiscryan/grpc-java,simonhorlick/grpc-java,madongfly/grpc-java,nmittler/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,rmichela/grpc-java,louiscryan/grpc-java,carl-mastrangelo/grpc-java,eonezhang/grpc-java,rmichela/grpc-java,LuminateWireless/grpc-java,LuminateWireless/grpc-java,anuraaga/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,elandau/grpc-java,LuminateWireless/grpc-java,ejona86/grpc-java,simonhorlick/grpc-java,elandau/grpc-java,zpencer/grpc-java,pieterjanpintens/grpc-java,zhangkun83/grpc-java,simonhorlick/grpc-java,pieterjanpintens/grpc-java,madongfly/grpc-java,elandau/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,ejona86/grpc-java,eonezhang/grpc-java,zpencer/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,simonhorlick/grpc-java,nmittler/grpc-java,zhangkun83/grpc-java,rmichela/grpc-java,dapengzhang0/grpc-java,nmittler/grpc-java,grpc/grpc-java,zpencer/grpc-java