Fix warning

atom.xml中の警告を修正

Build Warning: Layout 'nil' requested in atom.xml does not exist.
この警告が出るので nil -> nullに変更

Jekyll2.0から非推奨な設定の書き換え

Warning:
     Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.

@see mmistakes/hpstr-jekyll-theme#25