Adding compression

Piping to a stream to check the contents seems like overkill.  You could just do `.on('data', function(...`, though that might not be any cleaner.  Regardless, I think `es.mapSync` is simpler than `es.map` in this case:

```javascript
.pipe( es.mapSync( function(file) {
    expect( file.contents.length ).to.be.lessThan(
        originalSize / 2
    );
    done();
    return file;
} ) );
```