Bump Nokogiri version to 1.4.2

Wasabi uses Nokogiri's element_children method, which wasn't introduced
until 1.4.2.
Merge pull request #50 from getabetterpic/master

Bump Nokogiri version to 1.4.2

`#element_children` isn't available until Nokogiri v1.4.2, so that should really be the minimum version.