officially move to Go 1.4

@jbenet @whyrusleeping thoughts?

Godeps/Godeps.json

 {
 	"ImportPath": "github.com/jbenet/go-ipfs",
-	"GoVersion": "go1.3",
+	"GoVersion": "go1.4",
 	"Packages": [
 		"./..."
 	],