Changed the version check to not test for Go 1.5.

It now checks whether 6l exists, and if it does, errors. This should
cover all releases from 1.5 Beta onward. Closes #72.

cmd/util.go

diff --git a/cmd/util.go b/cmd/util.go
index c601322..2fc583e 100644
--- a/cmd/util.go
+++ b/cmd/util.go
@@ -7,7 +7,6 @@
 	"os/exec"
 	"path"
 	"path/filepath"
-	"strings"
 
 	"github.com/Masterminds/cookoo"
 )
@@ -36,11 +35,10 @@
 
 // VersionGuard ensures that the Go version is correct.
 func VersionGuard(c cookoo.Context, p *cookoo.Params) (interface{}, cookoo.Interrupt) {
-	cmd := exec.Command("go", "version")
+	// 6l was removed in 1.5, when vendoring was introduced.
+	cmd := exec.Command("go", "tool", "6l")
 	var out string
-	if out, err := cmd.CombinedOutput(); err != nil {
-		return nil, err
-	} else if !strings.Contains(string(out), "go1.5") {
+	if _, err := cmd.CombinedOutput(); err == nil {
 		Warn("You must install the Go 1.5 or greater toolchain to work with Glide.\n")
 	}
 	if os.Getenv("GO15VENDOREXPERIMENT") != "1" {