Gzipped file sizes don't match the output of `ember b -prod`

[al3xnag]

The reason is ember-cli uses zlib.gzip w/o any options, so default compression (Z_DEFAULT_COMPRESSION=6) is used, but ember-cli-bundlesize uses gzip-size w/o any options, which uses zlib under the hood with level=9 in that case.

Ideally, ember-cli-bundlesize should use the same compression level as ember-cli. If you want to avoid breaking changes, you can provide an option to configure it.