Received NoMethodError: undefined method `print_json'

(this is probably an edge case)

My project had stackprof on `0.2.12` in the `Gemfile.lock`. It turns out `print_json` is not defined in this version. It might help someone in the future if you update your gemspec to ensure a supported version of stackprof is installed:
```
spec.add_dependency 'stackprof', '>= 0.2.13'
```