Make compatible with SyliusThemeBundle 2.x (and so Sylius 1.9)

[maxperei]

Q	A
Branch?	master
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	yes
Related tickets	fixes #85
License	MIT

can you review @oallain plz :pray:

[maxperei]

to answer your previous question @oallain, i don't think so, because the only one assets/ folder — in this case — isn't related to any bundle and can be placed anywhere you want depending on the path mentioned in webpack.config.js:6 so according to me there ain't no need to change something here as long as manifest.json is properly configured...

[maxperei]

so @oallain, let me apologize because assets/ directory needed to be renamed as well to prevent public/_themes to be vanished during assets installation command

[maxperei]

no plan to make this works on the future