Upgrade to newer version of babel

Hi!

When building the current version of async (3.2.6) from source, I get several warnings about security issues in outdated packages. One potential way of addressing this is to use babel 7.x and `@rollup/plugin-node-resolve` instead. I have changed what is needed to get it to build on Debian (which no longer has some of the Node.js packages listed in `package.json`), and the diff is below. I'm not submitting it as a PR yet, as it will certainly also need changes to the `package.json` file and possibly elsewhere, but I could do so if you wished.

I also (locally) modified the test directory to allow for testing of the `dist/async.js` module (rather than the `lib/*` source files), and all the tests passed except for `should error when re-assigning event methods` in `test/queue.js`. I don't know if that is as a result of the changes below or an unrelated problem.

```diff
--- a/support/build/compile-module.js
+++ b/support/build/compile-module.js
@@ -2,9 +2,7 @@

const yargs = require('yargs');
const fs = require('fs');
-const {transformFile} = require('babel-core');
-const pluginCJS = require('babel-plugin-transform-es2015-modules-commonjs');
-const pluginModuleExports = require('babel-plugin-add-module-exports');
+const {transformFile} = require('@babel/core');

compileModule(yargs.argv, (err) => {
if (err) throw err;
@@ -13,8 +11,8 @@
function compileModule(options, callback) {
const {file, output} = options;
const plugins = [
- pluginModuleExports,
- pluginCJS
+ "babel-plugin-add-module-exports",
+ "@babel/plugin-transform-modules-commonjs"
];

transformFile(file, {
--- a/support/build/aggregate-bundle.js
+++ b/support/build/aggregate-bundle.js
@@ -1,5 +1,5 @@
const {rollup} = require('rollup');
-const nodeResolve = require('rollup-plugin-node-resolve');
+const nodeResolve = require('@rollup/plugin-node-resolve');

rollup({
input: 'build-es/index.js',
--- a/support/build/aggregate-module.js
+++ b/support/build/aggregate-module.js
@@ -1,5 +1,5 @@
const {rollup} = require('rollup');
-const nodeResolve = require('rollup-plugin-node-resolve');
+const nodeResolve = require('@rollup/plugin-node-resolve');

rollup({
input: 'build-es/index.js',
--- a/.babelrc
+++ b/.babelrc
@@ -1,7 +1,7 @@
{
"plugins": [
- "transform-es2015-modules-commonjs",
- "syntax-async-generators"
+ "@babel/plugin-transform-modules-commonjs",
+ "@babel/plugin-syntax-async-generators"
],
"env": {
"test": {
```