Compress + Mangle minified output.

Before:

```bash
ember build --environment=production
% ls -lah dist/assets/app.js
-rw-r--r--  1 rjackson  staff   781K May  1 22:24 dist/assets/app.js
```

After:

```bash
% ember build --environment=production
% ls -lah dist/assets/app.js
-rw-r--r--  1 rjackson  staff   523K May  1 22:26 dist/assets/app.js
```