Gem install can throw a ssl exception. This is a solution on Stackoverflow

Summary of the remedy:

1. Download

https://raw.githubusercontent.com/rubygems/rubygems/master/lib/rubygems/ssl_certs/AddTrustExternalCARoot-2048.pem

2. Find out installation location

    C:\>gem which rubygems
    C:/Ruby21/lib/ruby/2.1.0/rubygems.rb

3. Install the file

Then just copy the .pem file into ../2.1.0/rubygems/ssl_certs/