v0.6.0

• Use prop-types package to avoid warnings with react 15.5. Drop support for react < 15 (#28 Thanks @vmasto!)
• Use babel 6 to build the library. If requiring this library using require, you'll need to access the default propertly, like this:

// Don't do this:
// var Sound = require('react-sound');

// Instead, do this:
var Sound = require('react-sound').default;