Move kindlegen out of optional deps

[slonopotamus]

Users suffer (#268):

$ gem install asciidoctor-epub3 --pre
$ asciidoctor -b epub3 -r asciidoctor-epub3 -a ebook-format=kf8 book.adoc
cannot load such file -- kindlegen
  Use --trace for backtrace

@mojavelinux Do you have arguments against this?

[slonopotamus]

Aaaargh, we can't because it is conditional =/ I'm not sure how to fix this then.

[mojavelinux]

I'd go ahead and make it a regular dependency.

[slonopotamus]

If 3.0.3 is added to gemspec, it can't be installed on Windows (#213). If 3.0.5 is added to gemspec, we lose Ruby 2.3 support (tdtds/kindlegen@8c3b29c). And kindlegen maintainer already rejected lowering required Ruby version to 2.3.

[slonopotamus]

Okay, I think I figured out how to handle this: #272.

[mojavelinux]

The range is the correct thing to do. It allows the kindlegen gem to be installed first (or via Bundler), and installing this gem won't try to upgrade it.

You might not want to make the top of the range so specific. I think < 3.1.0 would be sufficient.

[slonopotamus]

kindlegen doesn't follow semantic versioning (they raised Ruby version from 2.3 to 2.4 in kindlegen-3.0.4), so I'd prefer to play safe here.

[slonopotamus]

This will be undone, see #288.