[PBXFileReference] Removes all items related to an external project.

Test case thanks to @jpsim.
[CHANGELOG] Don't use internal markdown links.

This should be readable in a text editor.
Merge branch 'jpsim-jp-remove-subproject-test'
Drop native extension
Travis: use ruby workers
Fix import
Plist: use libxml
Plist: Fix import
Plist: don't require libxml-ruby in the gemspec
Plist: move logic to a dedicate helper
Plist: robustness for plutil usage
PlistHelper: use native extension specs
PlistHelper: remove specs not relevant anymore
Bundle: install libxml-ruby on mac
PlistHelper: refine specs
Specs: compatibility when plutil is not available
Specs: robustness
Bundle: clean up
Project: clean up UUID generation
PlistHelper: add warning for libxml