Add 'cantarell-fonts' as a dependency of 'ttf-google-fonts-git'

I based this pull request on top of my earlier pull request (sebastianst/aur-packages#2), so you can do one of the following:
* discard pull request sebastianst/aur-packages#2 and accept this pull request
* accept pull request sebastianst/aur-packages#2 and then accept this pull request (I think this should work okay)
* accept pull request sebastianst/aur-packages#2 and discard this pull request

The first two options should merge all changes between both pull requests. The last option would merge in my various fixes for `ttf-google-fonts-git` without adding the dependency on `cantarell-fonts`. I added a dependency on `cantarell-fonts` to my AUR package `ttf-google-fonts-hg` about two weeks ago, and FredBezies says it worked well for him, so this change should be fine.