travis: Fix lint

Turns out `make lint` failures were not actually breaking the build
because we added an `|| echo "Skipping lint"` which would get called
when `make lint` failed.

This should cause lint failures as expected.

.travis.yml

@@ -15,5 +15,5 @@ cache:
 install:
   - make dependencies
 script:
-  - test "$LINT" -eq 1 && make lint || echo "Skipping lint"
+  - test -z "$LINT" && echo "Skipping lint" || make lint
   - make ci

Makefile

@@ -12,7 +12,9 @@ dependencies:
 	glide --version || go get -u -f github.com/Masterminds/glide
 	glide install
 	@echo "Installing uber-license tool..."
-	update-license || go get -u -f go.uber.org/tools/update-license
+	update-license -h || go get -u -f go.uber.org/tools/update-license
+	@echo "Installing golint..."
+	golint -h || go get -u -f github.com/golang/lint/golint
 
 .PHONY: lint
 lint: