ability to specify rails version on command line

Gemfile

@@ -3,7 +3,18 @@ source "http://rubygems.org"
 # Specify your gem's dependencies in devise_ip_filter.gemspec
 gemspec
 
-gem 'rails', '~> 3.2'
+rails_version = ENV["RAILS_VERSION"] || "default"
+
+rails = case rails_version
+        when "master"
+          {github: "rails/rails"}
+        when "default"
+          "~> 3.2"
+        else
+          "~> #{rails_version}"
+        end
+
+gem "rails", rails
 
 group :test do
   gem "sqlite3"