fixed multiple style guide violations

lib/code_ocean/config.rb

@@ -6,15 +6,14 @@ module CodeOcean
 
     def read(options = {})
       path = Rails.root.join('config', "#{@filename}.yml#{options[:erb] ? '.erb' : ''}")
-      if ::File.exists?(path)
+      if ::File.exist?(path)
         content = options[:erb] ? YAML.load(ERB.new(::File.new(path, 'r').read).result) : YAML.load_file(path)
         content[Rails.env].with_indifferent_access
       else
         fail(Error, "Configuration file not found: #{path}")
       end
     end
-  end
 
-  class Config::Error < RuntimeError
+    class Error < RuntimeError; end
   end
 end