Revert "Actually fixes #11. Resets the local RegExp engine to default."

This reverts commit b608d4cbdd2a3866fe03c1a583a77a6e08aca05d.

Setting a global option is not the right way to fix this problem, and
does not help for those using vim 7.3 or earlier.
Fix an unruly pattern in the YAML syntax file.

This fixes #11 and #12, at least for the reproduceable examples
discussed in those issues.