Update dependency installation for Fedora

[FranklinYu]

Latest Fedora uses dnf instead of yum, and rubygem-json is necessary. The line should be

sudo dnf group install 'Development Tools' && sudo dnf install curl file git python-setuptools rubygem-json

(Actually I suspect that curl, file, and git are installed by default, but that doesn't hurt anyway.)

[sjackman]

Thanks for the bug report, Franklin. If the ruby package is not installed at all, it should work just fine. Linuxbrew will install its own copy of Ruby. If however the ruby package is installed but rubygem-json is not installed, it may not work. I haven't tested that, but I've got a feeling it'll break. The two work arounds to fix it are…

1. sudo dnf install rubygem-json as you suggested
2. brew vendor-install ruby

A proper fix would need setup-ruby-path to detect whether Ruby has the json gem. A PR would be welcome to address this issue.
https://github.com/Linuxbrew/brew/blob/master/Library/Homebrew/utils/ruby.sh#L40

I've opened PR Linuxbrew/brew#506 to address the issue of yum vs dnf.