Gemfile: stable version with latest version of gems

[aollier]

This commit updates Gemfile to use the latest version of gems and makes a problem with bundle install disappear:

$ bundle install
Fetching https://github.com/asciidoctor/asciidoctor-epub3
error: cannot open .git/FETCH_HEAD: Permission denied
Retrying git fetch --force --quiet --tags "/home/<user>/.bundler/cache/git/asciidoctor-epub3-3986efa04ab5fd28abe5678d7e330e173e425f18" due to error (2/4): Bundler::Source::Git::GitCommandError Git error: command `git fetch --force --quiet --tags "/home/<user>/.bundler/cache/git/asciidoctor-epub3-3986efa04ab5fd28abe5678d7e330e173e425f18"` in directory /usr/share/gems/bundler/gems/asciidoctor-epub3-c8ca2e297cd1 has failed.
If this error persists you could try removing the cache directory '/home/<user>/.bundler/cache/git/asciidoctor-epub3-3986efa04ab5fd28abe5678d7e330e173e425f18'

See #1068

[aollier]

Issue 1068 is already closed, that's why I didn't use a keyword to close it. I closed issue 1068 because I finally found the problem I had: Bundler was too old (1.11.2), and after the upgrade to 2.1.2 I finally could build the book.
Updating Gemfile was not sufficient to solve the problem, but it may be part of the problem. For me, removing the cache did not resolve the problem. That's why I mention it (issue 1068) nevertheless.

OK it is probably not very clean but the essential is that it is mentionned; I explained all the problems and the issue is closed.

[HonkingGoose]

Ah yes, issue 1068 is closed, I hadn't noticed that yet... 🤓 Disregard my previous comments... 😄