An in-range update of uglify-js is breaking the build 🚨

## The devDependency [uglify-js](https://github.com/mishoo/UglifyJS2) was updated from `3.6.8` to `3.6.9`.

🚨 [View failing branch](https://github.com/jakubpawlowicz/clean-css/compare/master...jakubpawlowicz:greenkeeper%2Fuglify-js-3.6.9).

This version is **covered** by your **current version range** and after updating it in your project **the build failed**.




uglify-js is a devDependency of this project. It **might not break your production code or affect downstream projects**, but probably breaks your build or test tools, which may **prevent deploying or publishing**.



<details>
<summary>Status Details</summary>

- ❌ **continuous-integration/appveyor/branch:** AppVeyor build failed ([Details](https://ci.appveyor.com/project/jakubpawlowicz/clean-css/builds/28813694)).
- ❌ **continuous-integration/travis-ci/push:** The Travis CI build failed ([Details](https://travis-ci.org/jakubpawlowicz/clean-css/builds/611100076?utm_source=github_status&utm_medium=notification)).
</details>


---

<details>
<summary>Release Notes for v3.6.9</summary>

<p> </p>
</details>

<details>
<summary>Commits</summary>
<p>The new version differs by 6 commits.</p>
<ul>
<li><a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/commit/0d17c5b0fa3a4305177f9561ba1f05835a9ffea3"><code>0d17c5b</code></a> <code>v3.6.9</code></li>
<li><a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/commit/5b20bad4b360dc58d66be0143d739ac3cfad9bfe"><code>5b20bad</code></a> <code>fix corner case in <code>dead_code</code> (#3579)</code></li>
<li><a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/commit/765a06340fa7e9779701d1f9941c45fa5c8be819"><code>765a063</code></a> <code>enable cache on GitHub Actions (#3570)</code></li>
<li><a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/commit/5045e140b18485132b533c4ee52a18f97f3bf6f9"><code>5045e14</code></a> <code>fix corner case in <code>conditionals</code> (#3577)</code></li>
<li><a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/commit/10648c9af66364697f0f51219b0497fdfcb6dd34"><code>10648c9</code></a> <code>enhance <code>dead_code</code> (#3575)</code></li>
<li><a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/commit/87e67ec29958a4167d5c202ae3a0fe4be1e10faf"><code>87e67ec</code></a> <code>fix corner case in <code>collapse_vars</code> (#3574)</code></li>
</ul>
<p>See the <a href="https://urls.greenkeeper.io/mishoo/UglifyJS2/compare/61a0dad9fe9a39245ff89a6c01bac8f1788f3293...0d17c5b0fa3a4305177f9561ba1f05835a9ffea3">full diff</a></p>
</details>


<details>
<summary>FAQ and help</summary>

There is a collection of [frequently asked questions](https://greenkeeper.io/faq.html). If those don’t help, you can always [ask the humans behind Greenkeeper](https://github.com/greenkeeperio/greenkeeper/issues/new).
</details>

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