Make XSL tests less strict
Fix tests failing due to trailing whitespace
Make XML-related tests more robust
Skip safe level tests on JRuby
Skip Nokogiri tests on JRuby

Nokogiri on JRuby is unusably buggy. For details, see https://github.com/nanoc/nanoc/pull/422.
Fix broken assumption (JRuby-Nokogiri failure)
Print JRuby-Nokogiri warning
Use more reliable checksumming method

Marshal.dump is not consistent in its output.
Remove broken checksum test
Fix relativize_paths tests
Skip pruner test that checks behavior for output dir symlink

JRuby 1.7.11 has buggy File.find behavior. Also see
https://github.com/jruby/jruby/issues/1647
Fix wrong XSL filter test regexes
Fix raising piper errors
Fix wrong relativize_paths expectation
Use latest JRuby and Rubinius
Add separate Travis target for JRuby with Nokogiri disabled
Use listen only on 1.9+
Use mime-types only on 1.9+
Only use fog, handlebars, redcarpet and rubocop on 1.9+
Do not load Rubocop if unavailable