Fix mismatching variable names when using an underscore

This is a port of https://github.com/rails/cache_digests/pull/51

The ERBTracker template digest helper class was using a regex to match render calls and it was incorrectly not matching against variables with underscores in the name. This caused it to use the wrong regex match data to populate the template dependency. Because underscore is a valid character for a variable, this fixes the ERBTracker to match it properly.