Fix Rabl::Tracker to match paths with numbers

It fixes finding dependencies for paths with numbers like `partial("v1/posts")` etc. Currently in such case Rails outputs a a cryptic warning `Couldn't find template for digesting: v` and only for the first request that uses caching, so it's really easy to overlook it.

I haven't written any tests, because there are no tests for `Rabl::Tracker` yet and I couldn't figure out how to create a template that could be passed to tracker object. I tried creating a `RablTemplate`, but it complained that it doesn't have `source` method.