Add react/prefer-stateless-function to enforce stateless components written as a pure function

[ryansobol]

https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/prefer-stateless-function.md

Waiting on jsx-eslint/eslint-plugin-react#491 to land...

[ryansobol]

Which it did in https://github.com/yannickcr/eslint-plugin-react/blob/master/CHANGELOG.md#422---2016-03-14