ci: Manage lint decision in travis.yml

Rather than hard-coding the Go version we lint against in the Makefile,
this switches to managing that in the .travis.yml -- the same place we
manage the versions we're testing against.

.travis.yml

@@ -1,17 +1,23 @@
 language: go
 sudo: false
-go:
-  - 1.12.x
-  - 1.13.x
+
 go_import_path: go.uber.org/zap
 env:
   global:
     - TEST_TIMEOUT_SCALE=10
     - GO111MODULE=on
+
+matrix:
+  include:
+  - go: 1.12.x
+  - go: 1.13.x
+    env: LINT=1
+
 script:
-  - make lint
+  - test -z "$LINT" || make lint
   - make test
   - make bench
+
 after_success:
   - make cover
   - bash <(curl -s https://codecov.io/bash)

Makefile

@@ -13,22 +13,11 @@ GO_FILES := $(shell \
 	find . '(' -path '*/.*' -o -path './vendor' ')' -prune \
 	-o -name '*.go' -print | cut -b3-)
 
-# The linting tools evolve with each Go version, so run them only on the
-# latest stable release.
-GO_VERSION := $(shell go version | cut -d " " -f 3)
-GO_MINOR_VERSION := $(word 2,$(subst ., ,$(GO_VERSION)))
-LINTABLE_MINOR_VERSIONS := 13
-
-ifneq ($(filter $(LINTABLE_MINOR_VERSIONS),$(GO_MINOR_VERSION)),)
-SHOULD_LINT := true
-endif
-
 .PHONY: all
 all: lint test
 
 .PHONY: lint
 lint: $(GOLINT)
-ifdef SHOULD_LINT
 	@rm -rf lint.log
 	@echo "Checking formatting..."
 	@gofmt -d -s $(GO_FILES) 2>&1 | tee lint.log
@@ -41,9 +30,6 @@ ifdef SHOULD_LINT
 	@echo "Checking for license headers..."
 	@./checklicense.sh | tee -a lint.log
 	@[ ! -s lint.log ]
-else
-	@echo "Skipping linters on" $(GO_VERSION)
-endif
 
 $(GOLINT):
 	go install golang.org/x/lint/golint