Use jdiff instead of jsondiff

Summary:
jdiff seems the new name of jsondiff executable.

This fixes the following issue:

$ make test
[...]
run`'s command `jsondiff /tmp/tmp.CXo1sQNPui data/acmedns-merged.json`
exited with code 127, indicating 'Command not found'.

Reviewers: yousra

Reviewed By: yousra

Differential Revision: https://devcentral.nasqueron.org/D3978