[LRUG] Upgraded to Ruby 2.3.2 now rubygems doesn't work due to cert problems

[gvim]

I just upgraded, via rbenv, to Ruby 2.3.2 and now rubygems doesn't work 
due to:

ERROR:  While executing gem ... (Gem::RemoteFetcher::FetchError)
     SSL_connect returned=1 errno=0 state=error: certificate verify 
failed (https://api.rubygems.org/specs.4.8.gz)

I downloaded rubygems-update-2.6.7.gem and tried to install locally but 
this errors with:

ERROR:  While executing gem ... (TypeError)
     no implicit conversion of nil into String

... so now my Ruby is borked. Any suggestions how to fix this?

gvim