"kardianos/osext" is obsolete on go-1.8+

The following patch allows to remove redundant github.com/kardianos/osext dependency:

--- a/executors/shell/executor_shell.go
+++ b/executors/shell/executor_shell.go
@@ -8,9 +8,8 @@
 	"os/exec"
 	"path/filepath"
 
 	"fmt"
-	"github.com/kardianos/osext"
 	"github.com/sirupsen/logrus"
 	"gitlab.com/gitlab-org/gitlab-runner/common"
 	"gitlab.com/gitlab-org/gitlab-runner/executors"
 	"gitlab.com/gitlab-org/gitlab-runner/helpers"
@@ -126,9 +125,9 @@
 }
 
 func init() {
 	// Look for self
-	runnerCommand, err := osext.Executable()
+	runnerCommand, err := os.Executable()
 	if err != nil {
 		logrus.Warningln(err)
 	}
 
--- a/vendor/github.com/ayufan/golang-kardianos-service/example/runner/runner.go
+++ b/vendor/github.com/ayufan/golang-kardianos-service/example/runner/runner.go
@@ -13,9 +13,8 @@
 	"os"
 	"os/exec"
 	"path/filepath"
 
-	"github.com/kardianos/osext"
 	"github.com/kardianos/service"
 )
 
 // Config is the runner app config structure.
@@ -104,9 +103,9 @@
 	return nil
 }
 
 func getConfigPath() (string, error) {
-	fullexecpath, err := osext.Executable()
+	fullexecpath, err := os.Executable()
 	if err != nil {
 		return "", err
 	}
 
--- a/vendor/github.com/ayufan/golang-kardianos-service/service.go
+++ b/vendor/github.com/ayufan/golang-kardianos-service/service.go
@@ -65,9 +65,9 @@
 	"errors"
 	"fmt"
 	"path/filepath"
 
-	"github.com/kardianos/osext"
+	"os"
 )
 
 const (
 	optionKeepAlive            = "KeepAlive"
@@ -120,9 +120,9 @@
 func (c *Config) execPath() (string, error) {
 	if len(c.Executable) != 0 {
 		return filepath.Abs(c.Executable)
 	}
-	return osext.Executable()
+	return os.Executable()
 }
 
 var (
 	system         System