Here is a quick solution:
Go to your gem file and change
source 'https://rubygems.org'
to
source 'http://rubygems.org' #remove the 's'
Check this similar question on StackOverflow for more solutions and insights into the error :)
Edit:
Based on inputs by @Anthony in the comments below, here is a more permanent solution :
Download : AddTrustExternalCARoot-2048.pem.(Note: File must have .pem as extension. Browsers like Chrome will try to save it as plain text file. Ensure you change the filename to have .pem in it after you have downloaded it.)
Now Run the following in your command prompt:
C:\>gem which rubygems
Thiw will give you a path where you need to place the above .pem file. Navigate inside the directory to which the path points. Inside it locate ssl_certs directory and copy the .pem file we obtained from previous step inside.
This shall do the trick!
