Configure Bundler to validate task dependencies (#1242)

Rouge previously allowed Rake to be run without prepending the command
with `bundle exec`. The Rake task for updating the changelog has a
dependency that breaks this. This commit fixes that.

Rakefile

@@ -2,6 +2,7 @@
 
 require 'rake/clean'
 require 'pathname'
+require 'bundler/setup'
 require "bundler/gem_tasks"
 require "rake/testtask"