Add arel as a git source to Gemfile-edge

Active Record in Rails 5.1 will depend on the currently unreleased
Arel 8.0 so we need to include it as a GitHub source entry.

Author: pixeltrix

gemfiles/Gemfile-edge

@@ -1,5 +1,6 @@
-source 'https://rubygems.org'
+source "https://rubygems.org"
 
-gemspec path: '..'
+gemspec path: ".."
 
-gem 'rails', github: 'rails/rails', branch: 'master'
+gem "rails", github: "rails/rails", branch: "master"
+gem "arel", github: "rails/arel", branch: "master"