Update to allow using ractive 0.5.5

the ractive dependency is listed an `~0.4`. This is equivalent to `0.4.x` according to [semver](https://www.npmjs.org/doc/misc/semver.html), making the new 0.5.5 not available to ractivate.

This changes it to `0.x`. It should be fairly reasonable to expect that `Ractive.parse()` is more-or-less guaranteed to be the same-ish before `1.0.0`.

(Thanks for this package btw!)