[yaml] string with = and '{',',' break syntax highlighting

[berdario]

Related to #1550 and should probably be reported to https://github.com/textmate/yaml.tmbundle/ ...but given all the defects, it might just be better to adopt Atom's YAML grammar, since it doesn't have all these bugs

Here are a couple of examples:

- blue: a="brown,not_brown"
- not_blue: foo

and

- blue: foo="}"
- not_blue: 1

This is very common in ansible playbooks.