Jetpack::maybe_min_asset should check the SCRIPT_DEBUG value

The `SCRIPT_DEBUG` constant allows developers to [work with non-minified versions](https://github.com/Automattic/jetpack/blob/eaa39d4967b26e6098c7df03799bda4da06be93d/docs/development-environment.md#script_debug) of .js files. However, `JETPACK::maybe_min_asset` doesn't check the value of that constant before using a minified asset.

#### Steps to Reproduce
1. Install, activate, and connect Jetpack.
2. Enable lazy image loading.
3. Create a post.
4. Visit the post, open the browser inspector, and check the version of the `lazy-images.js` file. You should see the `lazy-images.min.js` file is used.
5. Set the `SCRIPT_DEBUG` constant to true.
6. Visit the post from step 3 again and check the browser inspector. The minified version is still used.

(I added the Bug label to this because the behavior was unexpected, but this issue only affects developers. Users are not affected.)