Allow the loading of a external Redmine environment

* If the REDMINE_ROOT environment variable is set, it will be used to load
  the Rails environment.  This will allow the plugin to be tested outside of
  a Redmine's vendor/plugins directory

spec/spec_helper.rb

@@ -1,7 +1,11 @@
 # This file is copied to ~/spec when you run 'ruby script/generate rspec'
 # from the project root directory.
 ENV["RAILS_ENV"] = "test"
-require File.expand_path(File.dirname(__FILE__) + "/../../../../config/environment")
+
+# Allows loading of an environment config based on the environment
+redmine_root = ENV["REDMINE_ROOT"] || File.dirname(__FILE__) + "/../../../.."
+require File.expand_path(redmine_root + "/config/environment")
+
 require 'spec'
 require 'spec/rails'