cucumber: don't manually include assertions

It causes hard to debug infinite loops and it's done automatically
in recent versions of Cucumber anyway.

features/support/env.rb

@@ -2,10 +2,6 @@ require 'fileutils'
 require 'rr'
 require 'test/unit'
 
-World do
-  include Test::Unit::Assertions
-end
-
 TEST_DIR    = File.join('/', 'tmp', 'jekyll')
 JEKYLL_PATH = File.join(File.dirname(__FILE__), '..', '..', 'bin', 'jekyll')