Add Go 1.12 for Travis (#707)

Makefile rules have to change for 1.12 support (can't use go tool vet).

Drops 1.10 as we only support current and previous release.

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 sudo: false
 go:
-  - 1.10.x
   - 1.11.x
+  - 1.12.x
 go_import_path: go.uber.org/zap
 env:
   global:

Makefile

@@ -9,7 +9,7 @@ PKG_FILES ?= *.go zapcore benchmarks buffer zapgrpc zaptest zaptest/observer int
 # stable release.
 GO_VERSION := $(shell go version | cut -d " " -f 3)
 GO_MINOR_VERSION := $(word 2,$(subst ., ,$(GO_VERSION)))
-LINTABLE_MINOR_VERSIONS := 10
+LINTABLE_MINOR_VERSIONS := 12
 ifneq ($(filter $(LINTABLE_MINOR_VERSIONS),$(GO_MINOR_VERSION)),)
 SHOULD_LINT := true
 endif
@@ -45,7 +45,7 @@ ifdef SHOULD_LINT
 	@echo "Installing test dependencies for vet..."
 	@go test -i $(PKGS)
 	@echo "Checking vet..."
-	@$(foreach dir,$(PKG_FILES),go tool vet $(VET_RULES) $(dir) 2>&1 | tee -a lint.log;)
+	@go vet $(VET_RULES) $(PKGS) 2>&1 | tee -a lint.log
 	@echo "Checking lint..."
 	@$(foreach dir,$(PKGS),golint $(dir) 2>&1 | tee -a lint.log;)
 	@echo "Checking for unresolved FIXMEs..."