fix: OpenStruct causes NameError in Rake13.2.0

Loading of "ostruct" depends on Rake, so we need to explicitly load it and resolve the dependency.
Because Rake does not load "ostruct" since version 13.2.0.
[ruby/rake#545](ruby/rake#545)

Author: janz93

lib/htmlcsstoimage.rb

@@ -1,6 +1,7 @@
 require "htmlcsstoimage/version"
 require "httparty"
 require "addressable"
+require "ostruct"
 
 class HTMLCSSToImage
   include HTTParty