fixing issue#225(deprecation warning)

I did following two things:
1,_config.yml; change "pygments: true" to "highlighter: pygments"
2,rss.xml&atom.xml; change "Layout: nil" to "Layout: null"

Comparing to pull request aimed at issue[#223](https://github.com/plusjade/jekyll-bootstrap/pull/223),instead of explicitly adding markdown intepreter (`markdown:kramdown`),  I did nothing because in practice I used 'redcarpet'  intepreter.I'd rather do nothing instead of doing something that I am not sure what consequence would occur.
Besides, I changed rss.xml and atom.xml.