Refactor Ruby Gem Formulas to use gem fetch instead of hard coded rubygems.org url

QA steps:
` brew tap sportngin/hombrew`
` brew edit sportngin-brew-gem `
```ruby
require 'formula'
class SportnginBrewGem < Formula
  homepage 'https://github.com/sportngin/brew-gem'
  head 'https://github.com/sportngin/brew-gem/archive/use-gem-fetch.tar.gz'

   def install
     bin.install 'bin/brew-gem'
   end
end
```
` brew install --HEAD sportngin-brew-gem`