Use v1.1 of the double-conversion library

Using a fixed version of the double-conversion library rather
then master prevents accidental future breakage.

rebar.config

@@ -30,5 +30,5 @@
 
 {deps, [
     {'double-conversion', ".*",
-        {git, "https://code.google.com/p/double-conversion/", "master"}, [raw]}
+        {git, "https://code.google.com/p/double-conversion/", "v1.1"}, [raw]}
 ]}.