Bump asciidoctor-epub3 to 1.5.0.alpha.14

[slonopotamus]

This PR doesn't depend on #1373.

[slonopotamus]

There's an interesting bug about your negative leveloffset. Asciidoctor has an rule about appendix:

the first subsection of the appendix must be a level-3 section, not a level-2 section. That’s because an appendix cannot contain chapters.

When you're applying -1 leveloffset, your appendix subsections become level-2 sections so they're not subsections of appendix anymore, but instead are standalone chapters.

[slonopotamus]

I suddenly understood that your Gemfile automatically uses latest 1.5.0.alpha.WHATEVER asciidoctor-epub3 version.

[slonopotamus]

But you still need to fix leveloffset.