Fix jQuery #13251: use slice instead of substring
Update Sizzle: fixes native slice usage
Fix broken link in CONTRIBUTING.md, close gh-1132.
Fix requirements and dependencies in README, close gh-1131.
Never use the XML as parsed by the XHR instance. Use raw text instead and let the ajax conversion logic do the trick. -20 min/gzipped. Fixes #13276. Unit test added.