travis: Only check gofmt on Go 1.12

Makefile

@@ -1,4 +1,4 @@
-# A Self-Documenting Makefile: http://marmelab.com/blog/2016/02/29/auto-documented-makefile.html
+GOVERSION := $(shell go version | cut -d ' ' -f 3 | cut -d '.' -f 2)
 
 .PHONY: check fmt lint test test-race vet test-cover-html help
 .DEFAULT_GOAL := help
@@ -12,11 +12,13 @@ test-race: ## Run tests with race detector
 	go test -race ./...
 
 fmt: ## Run gofmt linter
+ifeq "$(GOVERSION)" "12"
 	@for d in `go list` ; do \
 		if [ "`gofmt -l -s $$GOPATH/src/$$d | tee /dev/stderr`" ]; then \
 			echo "^ improperly formatted go files" && echo && exit 1; \
 		fi \
 	done
+endif
 
 lint: ## Run golint linter
 	@for d in `go list` ; do \