Update concurrent-ruby to version 1.3.7 (#889)

Author: depfu[bot]

Gemfile.lock

@@ -92,7 +92,7 @@ GEM
       rack-test (>= 0.6.3)
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
-    concurrent-ruby (1.3.6)
+    concurrent-ruby (1.3.7)
     connection_pool (3.0.2)
     crack (1.0.1)
       bigdecimal