Use xml2::xml_find_first() instead of deprecated xml2::xml_find_one()

The next version of xml2 will be deprecating xml_find_one() in favor of xml_find_first(). The new function is a drop in replacement for the old, and no longer issues a warning when more than one result would be returned.

This pull request also adds a Remotes: hadley/xml2 entry to install the development version of xml2. Once the new version of xml2 is released (2-4 weeks) this should be removed.