Rubocop: Style/Proc

- Use proc instead of Proc.new
...and use lambda instead of proc

lib/jekyll/configuration.rb

@@ -127,7 +127,7 @@ module Jekyll
       # Get configuration from <source>/_config.yml or <source>/<config_file>
       config_files = override.delete('config')
       if config_files.to_s.empty?
-        default = %w(yml yaml).find(Proc.new { 'yml' }) do |ext|
+        default = %w(yml yaml).find(-> { 'yml' }) do |ext|
           File.exist?(Jekyll.sanitized_path(source(override), "_config.#{ext}"))
         end
         config_files = Jekyll.sanitized_path(source(override), "_config.#{default}")