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() {
+describe('<Avatar />', () => {
-  it('renders an img with the given src', function() {
+  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);

spec/javascript/components/button.test.jsx

@@ -4,8 +4,8 @@ import sinon from 'sinon';
 
 import Button from '../../../app/assets/javascripts/components/components/button'
 
-describe('<Button />', function() {
+describe('<Button />', () => {
-  it('simulates click events', function() {
+  it('simulates click events', () => {
     const onClick = sinon.spy();
     const wrapper = shallow(<Button onClick={onClick} />);
     wrapper.find('button').simulate('click');

spec/javascript/components/loading_indicator.test.jsx

@@ -3,8 +3,8 @@ import { shallow } from 'enzyme';
 
 import LoadingIndicator from '../../../app/assets/javascripts/components/components/loading_indicator'
 
-describe('<LoadingIndicator />', function() {
+describe('<LoadingIndicator />', () => {
-  it('renders text that indicates loading', function() {
+  it('renders text that indicates loading', () => {
     const wrapper = shallow(<LoadingIndicator />);
     expect(wrapper.text()).to.match(/loading/i);
   });