Add go-cmp to install-tools target (#528)

Use the general install-tools target so no one gets the vet failure if they follow the CONTRIBUTING.md instructions. Currently first attempt will fail with the download of go-cmp.

Author: Paulo Janotti

Makefile

@@ -38,7 +38,7 @@ test:
 	$(GOTEST) $(GOTEST_OPT) $(ALL_PKGS)
 
 .PHONY: travis-ci
-travis-ci: fmt install-go-cmp vet lint test-with-cover
+travis-ci: fmt vet lint test-with-cover
 
 .PHONY: test-with-cover
 test-with-cover:
@@ -85,9 +85,6 @@ vet:
 .PHONY: install-tools
 install-tools:
 	go get golang.org/x/lint/golint
-
-.PHONY: install-go-cmp
-install-go-cmp:
 	go get -u github.com/google/go-cmp/cmp
 
 .PHONY: agent