Allow OpenStructs to be used in this project

Since the performance hit would be minimal for a short-running process. This also fixes the build, which is currently broken.

From https://www.rubydoc.info/gems/rubocop/0.61.1/RuboCop/Cop/Performance/OpenStruct:

This cop checks for OpenStruct.new calls. Instantiation of an OpenStruct invalidates Ruby global method cache as it causes dynamic method definition during program runtime. This could have an effect on performance, especially in case of single-threaded applications with multiple OpenStruct instantiations.