ERROR: cannot verify www.opscode.com’s certificate

I am using OSS Chef 11.0.4 on Ubuntu 12.04 LTS , while running Knife ec2 create command I am getting below error

Bootstrapping Chef on ec2-xx-xx-xxx-xx.compute-1.amazonaws.com

  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com --2014-02-20 12:26:15--  https://www.opscode.com/chef/install.sh
  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com Resolving www.opscode.com (www.opscode.com)... 184.106.28.90
  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com Connecting to www.opscode.com (www.opscode.com)|184.106.28.90|:443... connected.
  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com ERROR: cannot verify www.opscode.com's certificate, issued by ‘/C=US/O=DigiCert Inc/CN=DigiCert Secure Server CA’:
  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com   Unable to locally verify the issuer's authority.
  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com To connect to www.opscode.com insecurely, use `--no-check-certificate'.
  ec2-xx-xx-xxx-xx.compute-1.amazonaws.com bash: line 88: chef-client: command not found

Help is appreciated.