fix hiera.yml for 1.9.x yaml parser

The yaml parser in ruby 1.9 and 2.0 doesn't believe that strings can start with
an unquoted `%` and will explode with a `found character that cannot start any
token while scanning for the next token` error.
Merge pull request #168 from richardc/yaml_quote

fix hiera.yml for 1.9.x yaml parser