use ES6 arrow functions

spec/javascript/components/avatar.test.jsx

@@ -3,8 +3,8 @@ import { render } from 'enzyme';
 
 import Avatar from '../../../app/assets/javascripts/components/components/avatar'
 
-describe('<Avatar />', function() {
-  it('renders an img with the given src', function() {
+describe('<Avatar />', () => {
+  it('renders an img with the given src', () => {
     const src = '/path/to/image.jpg';
     const wrapper = render(<Avatar src={src} size={100} />);
     expect(wrapper.find(`img[src="${src}"]`)).to.have.length(1);