Use a vendor directory for repeatable builds - fixes #816

This is using godep to manage the vendor directory.

vendor/github.com/skratchdot/open-golang/open/exec_darwin.go

@@ -0,0 +1,15 @@
+// +build darwin
+
+package open
+
+import (
+	"os/exec"
+)
+
+func open(input string) *exec.Cmd {
+	return exec.Command("open", input)
+}
+
+func openWith(input string, appName string) *exec.Cmd {
+	return exec.Command("open", "-a", appName, input)
+}