Fix various JRuby failures

This improves compatibility with nanoc on JRuby (and, to a lesser extent, Ruby 1.8.x).

Some changes:

* JRuby tests are run twice: once with Nokogiri enabled, and once with Nokogiri disabled. JRuby-Nokogiri has several problems (most of them being worked on), but I’d much rather split up the tests while waiting for Nokogiri to be fixed.

* A warning message is printed when using Nokogiri on JRuby. See above for the reasoning why.

* JRuby with 1.8 mode has been dropped. If you already have JRuby, why would you want to run it in 1.8 mode?

* The Gemfile now more clearly defines supported platforms for dependencies. It has been modified to accomodate both JRuby and Ruby 1.8.x.

* Temp path creation is more reliable on JRuby (JRuby seems to delete empty temp directories, which is sensible, but breaks an assumption made by nanoc).