[eslint config] [patch] class-methods-use-this: exempt React getChildContext

Added `getChildContext` to the exceptions list for `class-methods-use-this`

packages/eslint-config-airbnb/rules/react.js

@@ -23,6 +23,7 @@ module.exports = {
         'render',
         'getInitialState',
         'getDefaultProps',
+        'getChildContext',
         'componentWillMount',
         'componentDidMount',
         'componentWillReceiveProps',