chore: Work around an annoying rubocop change (GoogleCloudPlatform#215)

Author: dazuma

.rubocop.yml

@@ -31,3 +31,5 @@ Naming/FileName:
 Style/BlockDelimiters:
   Exclude:
     - "test/**/test_*.rb"
+Style/DoubleNegation:
+  Enabled: false

lib/functions_framework/server.rb

@@ -297,7 +297,7 @@ def show_error_details= show_error_details
           if show_error_details.nil?
             !::ENV["FUNCTION_DETAILED_ERRORS"].to_s.empty?
           else
-            show_error_details ? true : false
+            !!show_error_details
           end
       end