migrate lint script to a mage target

.travis.yml

@@ -10,7 +10,7 @@ install:
   - ./travis/install.sh
 script:
   - go run mage.go -v crossBuild
-  - ./travis/lint.sh
+  - go run mage.go lint
   - export GOMAXPROCS=4
   - export GORACE=halt_on_error=1
   - go test -race -v ./...

magefile.go

@@ -5,6 +5,8 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"os"
+	"path"
 
 	"github.com/magefile/mage/mg"
 	"github.com/magefile/mage/sh"
@@ -58,3 +60,12 @@ func CrossBuild() error {
 	}
 	return nil
 }
+
+func Lint() error {
+	gopath := os.Getenv("GOPATH")
+	if gopath == "" {
+		return fmt.Errorf("cannot retrieve GOPATH")
+	}
+
+	return sh.Run(path.Join(gopath, "bin", "golangci-lint"), "run", "./...")
+}

travis/lint.sh

@@ -1,7 +0,0 @@
-#!/bin/bash
-
-if [[ "$TRAVIS_GO_VERSION" =~ ^1\.15\. ]] && [[ "$TRAVIS_OS_NAME" == "linux" ]] && [[ "$GO111MODULE" == "on" ]]; then
-    $(go env GOPATH)/bin/golangci-lint run ./...
-else
-    echo "linter has not been run"
-fi