[ruby/rdoc] Use HEAD of kpeg

I compared the results between the latest release of kpeg gem (1.1.0)
and HEAD of kpeg gem with `time` command, and the execution time is
reduced from 38.35s sec to 35.06 sec.

ref. https://github.com/evanphx/kpeg/pull/43

https://github.com/ruby/rdoc/commit/682bcb48ce

lib/rdoc/Gemfile

@@ -5,7 +5,7 @@ gemspec
 group :development do
   gem 'rake'
   gem 'racc', '> 1.4.10'
-  gem 'kpeg'
+  gem 'kpeg', github: 'evanphx/kpeg'
   gem 'test-unit'
   gem 'minitest' # for test_rdoc_rubygems_hook.rb
   gem 'rubocop'