Update hash algorithm to sha256

[jgcaruso]

This PR updates the hashing algorithm to sha256 which is now supported by Gravatar.com

Testing

1. pull pr
2. run the following command to test the exported functions using your email address, and verify that the correct image is displayed in the browser
   • prints gravatar url node -e "var gravatar = require('./index.js'); console.log(gravatar.url('[email protected]'))"
   • prints a valid gravatar url with image size at 20px node -e "var gravatar = require('./index.js'); console.log(gravatar.url('[email protected]', {s: 20}))"