Move rouge loading

When generating a new site with `2.0.0.alpha.2`, I get the following error:

```
$ jekyll new benhanzl.com
/Users/benhanzl/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- rouge (LoadError)
	from /Users/benhanzl/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll/converters/markdown/redcarpet_parser.rb:42:in `<module:WithRouge>'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll/converters/markdown/redcarpet_parser.rb:41:in `<class:RedcarpetParser>'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll/converters/markdown/redcarpet_parser.rb:4:in `<class:Markdown>'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll/converters/markdown/redcarpet_parser.rb:3:in `<module:Converters>'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll/converters/markdown/redcarpet_parser.rb:2:in `<module:Jekyll>'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll/converters/markdown/redcarpet_parser.rb:1:in `<top (required)>'
	from /Users/benhanzl/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/benhanzl/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll.rb:11:in `block in require_all'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll.rb:10:in `each'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll.rb:10:in `require_all'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/lib/jekyll.rb:61:in `<top (required)>'
	from /Users/benhanzl/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/benhanzl/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/gems/jekyll-2.0.0.alpha.2/bin/jekyll:6:in `<top (required)>'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/bin/jekyll:23:in `load'
	from /Users/benhanzl/.rvm/gems/ruby-2.1.1@benhanzl/bin/jekyll:23:in `<main>'
```

This is caused because `rouge` is being required regardless of whether or not it is being used. I moved the `require` to when the renderer is setup.

I believe this relates to #2079.