bump to 0.1.5, this allows the use of the new hmac function

Author: fatboypunk

README.md

@@ -9,7 +9,7 @@ add
 ```ex
 def deps do
   [
-    {:lti, "~> 0.1.4"}
+    {:lti, "~> 0.1.5"}
   ]
 end
 ```

mix.exs

@@ -12,7 +12,7 @@ defmodule LTI.Mixfile do
       elixirc_paths: ["lib"],
       source_url: "https://github.com/defactosoftware/lti",
       start_permanent: true,
-      version: "0.1.4"
+      version: "0.1.5"
     ]
   end