jsonlint should look for JSON files at their post-re-organisation locations

[petertseng]

#351 moved files from the root of this repo to per-problem subdirectories under exercises/.

However, jsonlint still searches for files in the root of this repo only because its glob at https://github.com/exercism/x-common/blob/master/bin/jsonlint#L4 is *.json.

This means that we are not getting JSON validation on any JSON files currently.

[kytrinyx]

Ah, good catch. I'll fix that now.