Improve $LOADED_FEATURES append pattern

```start_with?('json')``` is not correct implementation.
For example, this prevents ```json_spec``` gem from being loaded.
Regular expression which checks if there is a file or folder starting
with json works better.
Improve $LOADED_FEATURES: fix merge hiccup
Improve $LOADED_FEATURES: do not mess with return
Merge pull request #157 from krasnoukhov/loaded-features

Improve $LOADED_FEATURES append pattern