Remove use of 'zip' gem dependencies

Remove parts of code that depends on of 'zip' gem instead of rubyzip:
* Zip::ZipEntry -> Zip::Entry
* Zip::ZipOutputStream -> Zip::OutputStream
* Zip::ZipInputStream -> Zip::InputStream

Here's the error stacktrace I had when generating:

/home/vincent/.rvm/gems/ruby-2.1.0/gems/zip-2.0.2/lib/zip/zip.rb:959:in `put_next_entry': wrong number of arguments (4 for 1..2) (ArgumentError)
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/gepub-0.6.4.6/lib/gepub/book.rb:240:in `write_to_epub_container'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/gepub-0.6.4.6/lib/gepub/book.rb:272:in `block in generate_epub'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/zip-2.0.2/lib/zip/zip.rb:942:in `open'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/gepub-0.6.4.6/lib/gepub/book.rb:270:in `generate_epub'
	from /home/vincent/git/sitepub/lib/sitepub/generators/epub_writer.rb:40:in `create_epub'
	from /home/vincent/git/egen/lib/tasks/generator.thor:14:in `generate_epub'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/runner.rb:36:in `method_missing'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/command.rb:29:in `run'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/command.rb:128:in `run'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /home/vincent/.rvm/gems/ruby-2.1.0/gems/thor-0.18.1/bin/thor:6:in `<top (required)>'
	from /home/vincent/.rvm/gems/ruby-2.1.0/bin/thor:23:in `load'
	from /home/vincent/.rvm/gems/ruby-2.1.0/bin/thor:23:in `<main>'
	from /home/vincent/.rvm/gems/ruby-2.1.0/bin/ruby_executable_hooks:15:in `eval'
	from /home/vincent/.rvm/gems/ruby-2.1.0/bin/ruby_executable_hooks:15:in `<main>'