Downgrade connection_pool version

Downgrade connection_pool from 3.0.2 to 2.5.5

Author: lodewiges

Gemfile.lock

@@ -119,7 +119,7 @@ GEM
     coderay (1.1.3)
     colorize (1.1.0)
     concurrent-ruby (1.3.6)
-    connection_pool (3.0.2)
+    connection_pool (2.5.5)
     crass (1.0.6)
     cssbundling-rails (1.4.3)
       railties (>= 6.0.0)