[tern addon] Use displayName if it exists for completion display text

Tern completion for required module returns name (module wuith quote)
and displayName (module without quote). This PR gives the capability
to display completion for required module without quote.

addon/tern/tern.js

@@ -216,7 +216,7 @@
         var completion = data.completions[i], className = typeToIcon(completion.type);
         if (data.guess) className += " " + cls + "guess";
         completions.push({text: completion.name + after,
-                          displayText: completion.name,
+                          displayText: completion.displayName || completion.name,
                           className: className,
                           data: completion});
       }