I've heard nothing about JunOS moving to Linux. That would be a truly bizarre move on their part, both in terms of technical quality of Linux vs FreeBSD, and licensing headaches with the GPL.
I found an article[0] from the end of 2014 that mentions they are using linux for some Open Compute project based switches (OCX models), as well as Yocto Linux as KVM hypervisor for JunOS on some models.
[0] http://www.enterprisenetworkingplanet.com/nethub/juniper-emb...