Use ES6 default properties instead of deprecated defaultProps property

Fixes Iconscout#86 and Iconscout#64

The work on this PR was sponsored by [SUSI&James GmbH](https://susiandjames.com/)

build/generate.js

@@ -29,8 +29,7 @@ uniconsConfig.forEach(icon => {
   const template = `import React from 'react';
 import PropTypes from 'prop-types';
 
-const ${name} = (props) => {
-  const { color, size, ...otherProps } = props
+const ${name} = ({ color = 'currentColor', size = '24', ...otherProps }) => {
   return React.createElement('svg', {
     xmlns: 'http://www.w3.org/2000/svg',
     width: size,
@@ -48,11 +47,6 @@ ${name}.propTypes = {
   size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
 };
 
-${name}.defaultProps = {
-  color: 'currentColor',
-  size: '24',
-};
-
 export default ${name};`
 
   fs.writeFileSync(location, template, 'utf-8')