Maintenance: Update dependency pg to v1.5.8

Gemfile.lock

@@ -437,7 +437,7 @@ GEM
     parser (3.3.5.0)
       ast (~> 2.4.1)
       racc
-    pg (1.5.7)
+    pg (1.5.8)
     power_assert (2.0.3)
     promise.rb (0.7.4)
     pry (0.14.2)