Don't load tasks if they're already loaded

We were having problems under certain conditions with Travis and
Knapsack where tasks were still being loaded twice and so they were
being executed twice.

spec/rails_helper.rb

@@ -12,7 +12,7 @@ require 'capybara/rails'
 require 'capybara/rspec'
 require 'selenium/webdriver'
 
-Rails.application.load_tasks
+Rails.application.load_tasks if Rake::Task.tasks.empty?
 I18n.default_locale = :en
 
 include Warden::Test::Helpers