Ruby 1.9.3 uses Psych as its Yaml parser, and 1.8.7 used the older and obsolete Syck. The issue you’re seeing is a known issue in LibYaml, the library that Psych is built on, and is due to some ambiguity that arises when colons are allowed in flow context.
There’s a Ruby bug about this.
The solution is to change your Yaml to use the block context:
order:
- :year
- :month
- :day
Rails has changed the template that generated this Yaml to avoid this bug.