Updated hypothesis key generation to be the same as sherpa (#226)

@@ -60,9 +60,9 @@ struct Hypothesis {
   std::string Key() const {
     // TODO(fangjun): Use a hash function?
     std::ostringstream os;
-    std::string sep = "-";
+    std::string sep;
     for (auto i : ys) {
-      os << i << sep;
+      os << sep << i;
       sep = "-";
     }
     return os.str();