Delete Gemfile.lock

After Oct 8th, started having problems 

stdout: Fetching source index from https://rubygems.org/
Could not verify the SSL certificate for
https://s3.amazonaws.com/production.s3.rubygems.org/quick/Marshal.4.8/multi_json-1.10.1.gemspec.rz.
There is a chance you are experiencing a man-in-the-middle attack, but most
likely your system doesn't have the CA certificates needed for verification. For
information about OpenSSL certificates, see bit.ly/ruby-ssl. To connect without
using SSL, edit your Gemfile sources and change 'https' to 'http'.

Found that removing Gemfile.lock and running bundle install fixed this.  Not sure why but committing this workaround.