`Pkg` fails to clone registry (SSL cert. error)


Using Julia 1.2 (happens also in 1.0.3), when running Pkg.update() I get:

(v1.2) pkg> update
   Cloning default registries into `~/.julia`
   Cloning registry from "https://github.com/JuliaRegistries/General.git"
ERROR: failed to clone from https://github.com/JuliaRegistries/General.git, error: GitError(Code:ECERTIFICATE, Class:SSL, the SSL certificate is invalid: 0x08 - The certificate is not correctly signed by the trusted CA)

I’ve seen this issue happen before on Windows, but I run Ubuntu 16 (behind a proxy)

Any ideas?