Fix JRuby-related Gemfile platforms

With C extensions disabled, a bunch of dependencies failed to compile.
Fix broken XSLT stylesheet

XSLT stylesheets that use variable names must define the variables.
libxml appears to be too permissive and allow broken stylesheets, while
JRuby-Nokogiri correctly fails to parse the stylesheet.

For details, see the following analysis of this issue by @headius:
https://github.com/sparklemotion/nokogiri/issues/1078#issuecomment-40936310

This test still fails due to a missing <?xml> header. For details, see
https://github.com/sparklemotion/nokogiri/issues/1079