drop present?

`/Users/mrose/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/bundler/gems/roo-60b0a57da1b7/lib/roo/excelx.rb:554:in 'cells_for_row_element': undefined method 'present?' for #<Nokogiri::XML::Element:0x007fdbb1db58c8> (NoMethodError)`

The gem was failing when parsing XLSX due to the present? not being a valid method on the Nokogiri::XML::Element object.

Dropped present? and things worked smoothly. Wish your fork would get merged into roo master!