revert Makefile change so prow works

Author: estroz

Makefile

@@ -37,13 +37,14 @@ else
 	$(Q)go mod vendor
 endif
 
-mod-update:
+# TODO: change to mod-update once prow CI command has changed.
+dep-update:
 	$(Q)go get -u -v
 
 clean:
 	$(Q)rm -rf build
 
-.PHONY: all test format mod clean
+.PHONY: all test format mod dep-update clean
 
 install:
 	$(Q)go install -gcflags "all=-trimpath=${GOPATH}" -asmflags "all=-trimpath=${GOPATH}" $(BUILD_PATH)