Bump concurrent-ruby from 1.3.6 to 1.3.7 (#64)

Author: dependabot[bot]

Gemfile.lock

@@ -29,7 +29,7 @@ GEM
     ast (2.4.3)
     base64 (0.3.0)
     bigdecimal (4.1.0)
-    concurrent-ruby (1.3.6)
+    concurrent-ruby (1.3.7)
     connection_pool (3.0.2)
     csv (3.3.5)
     date (3.5.1)