Update calibre.rb

[peterlewis]

Updated Calibre to latest version, replacing 'latest' as although 'latest' installs without a hitch, updates to calibre are not detected due to the version not changing.

[vitorgalvao]

The latest issue with Calibre has been discussed. I’ve made some points there (the second and sixth comments) as to why I think having it is a superior alternative (bottom line, it’s way easier to actually do an update and the cask does not break constantly).

If you still feel having the version is a better option, it’d be good to get your input as to why.

[peterlewis]

Thanks for that, I didn't see the previous discussion thread. Really insightful discussion! You've made me doubt my own opinion of it not following the 'traditional' Homebrew updating, due to most apps having auto-update built-in.

[vitorgalvao]

Yes, a huge amount of apps that auto-update are a significant difference between homebrew-cask and homebrew, which makes them require a different approach. We also have a fair share of apps with very specific use cases, like brackets, but we always try to make them consistent, instead of catering to every oddness.

Thank you for closing the pull request yourself, looking forward to future contributions.