Fix bug when using caching with 'rabl' gem.

The `template` option is used when initializing the
`Rabl::Engine` and used in `cache_key_with_digest` later on.
If not given on instantiation, the exception below is raised
from `cache_key_with_digest` when instantiating the `Digestor`.

    undefined method `gsub' for nil:NilClass
Merge pull request #136 from tuvistavie/master

Fix bug when using caching with 'rabl' gem.