Don't indicate git ref if master branch

lib/github.js

@@ -52,6 +52,12 @@ const splittedURL = fullURL => {
     ref = ref.substring(0, 7)
   }
 
+  // We're deploying master by default,
+  // so there's no need to indicate it explicitly
+  if (ref === 'master') {
+    ref = ''
+  }
+
   return {main, ref}
 }