features: Update the way we access Minitest assertions

Per https://github.com/cucumber/cucumber/wiki/Using-MiniTest.

features/step_definitions/jekyll_steps.rb

@@ -27,7 +27,9 @@ After do
   Dir.chdir(File.dirname(TEST_DIR))
 end
 
-World(Minitest::Assertions)
+World do
+  MinitestWorld.new
+end
 
 Given /^I have a blank site in "(.*)"$/ do |path|
   FileUtils.mkdir_p(path) unless File.exist?(path)

features/support/env.rb

@@ -1,8 +1,17 @@
 require 'fileutils'
 require 'posix-spawn'
-require 'minitest/assertions'
+require 'minitest/spec'
 require 'time'
 
+class MinitestWorld
+  extend Minitest::Assertions
+  attr_accessor :assertions
+
+  def initialize
+    self.assertions = 0
+  end
+end
+
 JEKYLL_SOURCE_DIR = File.dirname(File.dirname(File.dirname(__FILE__)))
 TEST_DIR    = File.expand_path(File.join('..', '..', 'tmp', 'jekyll'), File.dirname(__FILE__))
 JEKYLL_PATH = File.expand_path(File.join(File.dirname(__FILE__), '..', '..', 'bin', 'jekyll'))