Added test for nil in Roo::Excelx#read_hyperlinks to avoid error "NoMethodError (undefined method `text' for nil:NilClass)"
Updated the XPATH query to "/xmlns:worksheet/xmlns:hyperlinks/xmlns:hyperlink[id]" to avoid the previous error "NoMethodError (undefined method `text' for nil:NilClass)"
Merge pull request #123 from rui-castro/master

Fixes "NoMethodError (undefined method `text' for nil:NilClass)" issue when reading xlsx file (https://github.com/Empact/roo/issues/122)