Remove test for XML YAML parsing

The support for YAML parsing in XML has been removed from Active Support
since it introduced an security risk. See 8133a81 for more detail.
Merge pull request #8835 from sikachu/3-1-stable-fix-ars

Remove test for XML YAML parsing
Merge pull request #5896 from sferik/revert_5861

Revert #5861. Feature-detect which MultiJson API to use.
Conflicts:
	activesupport/activesupport.gemspec

This backports multi_json version depedency changes as applied.

Rationale: #5861

Patch by sferik
Merge pull request #8846 from AlexRiedler/revert_5861

Backport multi_json dependency revert of #5861 to 3-1-stable