To completely detail out the summary of all the above answers.
This problem is occuring because git cannot complete the https handshake with the git server were the repository you are trying to access is present.
Steps to get the certificate from the github server
Steps to add the certificate to local git certificate store
Now open the certicate you saved in the notepad and copy the content along with --Begin Certificate-- and --end certificate--
To find the path were all the certificates are stored for your git, execute the following command in cmd.
git config --list
Check for the key 'http.sslcainfo', the correspondig value will be path.
Now open 'ca-bundle.crt' present in that path.
Note 1 : open this file administrator mode otherwise you will not be able to save it after update. (Tip - you can use Notepad++ for this purpose)
Note 2 : Before modifying this file please keep a backup elsewhere.