Fix #171: RSA public exponent must be prime number

* 65537 is a well known prime number. ref: http://en.wikipedia.org/wiki/65537_(number)
* 2048 bits (256 bytes) is default size in ssh-keygen.
Merge pull request #172 from worksap-ate/ec2-keypair

Fix #171: RSA public exponent must be a prime number