Updates GUI text for new Homebrew version

[thatrobotdev]

The instructions in the GUI for downloading the kindle-comic-creator cask utilize an old brew command, so this PR updates the GUI text with the command that Homebrew uses now. It also renames references to "Brew" to "Homebrew".

---

This is what current homebrew tells you if you use the old command that the GUI suggests:

jameskerrane@Jamess-MBP ~> brew cask install kindle-comic-creator
Error: `brew cask` is no longer a `brew` command. Use `brew <command> --cask` instead.

This is a successful installation with the command that I changed the GUI instructions to:

jameskerrane@Jamess-MBP ~ [1]> brew install --cask kindle-comic-creator
==> Downloading https://kc2.s3.amazonaws.com/KindleComicCreatorInstall.dmg
Already downloaded: /Users/jameskerrane/Library/Caches/Homebrew/downloads/29bff483a8f694859b20eacf3cd654d0565c07fc31165f9ba8408c3f88edb26a--KindleComicCreatorInstall.dmg
Warning: No checksum defined for cask 'kindle-comic-creator', skipping verification.
==> Installing Cask kindle-comic-creator
==> Running installer for kindle-comic-creator; your password may be necessary.
Package installers may write to any location; options such as `--appdir` are ignored.
Password:
installer: Package name is Kindle Comic Creator
installer: Installing at base path /
installer: The install was successful.
🍺  kindle-comic-creator was successfully installed!

[axu2]

@thatrobotdev

Just a nitpick, since you can't copy paste from the UI, might be better to change the wording and link to a wiki page instead.

[thatrobotdev]

A direct link to this wiki page might be nice, although for users already familiar with Homebrew I appreciate the cask name being mentioned in the app- https://github.com/ciromattia/kcc/wiki/Installation#mac

Maybe something like, "Cannot find Kindle Comic Creator! Install the 'kindle-comic-creator' cask using Homebrew to enable MOBI conversion".

It looks like the original link to KindleGen is defunct, so this would also (solve?) that issue. (I'm confused reading through these issues, is it still recommended to install KindleGen from an unauthorized/archived source? #449 (comment))

Relevant lines

kcc/kindlecomicconverter/KCC_gui.py

Lines 746 to 751 in 5902d88

	self.addMessage('Cannot find <a href="http://www.amazon.com/gp/feature.html?ie=UTF8&docId='
	'1000765211"><b>KindleGen</b></a>! MOBI conversion is unavailable!', 'error')
	if sys.platform.startswith('win'):
	self.addMessage('Download it and place EXE in KCC directory.', 'error')
	elif sys.platform.startswith('darwin'):
	self.addMessage('Install it using <a href="http://brew.sh/">Homebrew</a>.', 'error')

[darodi]

@thatrobotdev

I made a PR:

• refactor display when kindlegen is missing #555

Can you tests if this is ok for you?
I'll merge it.