I managed to identify the origin of the problem, simplecov introduced branches coverage in 0.18 and modified the format of the .json file.
You can see here an interesting discussion in another project impacted by the same problem.
There is a ticket to track this issue.
The only workaround that I see currently is to use a previous version of simplecov (<= 0.17.x).
Thanks for spotting this problem.
Quentin