diff --git a/appengine/flexible/static_files/Gemfile.lock b/appengine/flexible/static_files/Gemfile.lock
index f755a54b6..49d464bc2 100644
--- a/appengine/flexible/static_files/Gemfile.lock
+++ b/appengine/flexible/static_files/Gemfile.lock
@@ -12,7 +12,7 @@ GEM
       rack-test (>= 0.6.3)
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
-    concurrent-ruby (1.2.2)
+    concurrent-ruby (1.3.7)
     cuprite (0.14.3)
       capybara (~> 3.0)
       ferrum (~> 0.13.0)