Instruct removing Sprockets from Gemfile

[sedubois]

I suppose the upgrade guide introduced in #35 should instruct to remove Sprockets from the Gemfile?

Section 3 point 4 instructs to remove app/assets/config/manifest.js, which yields Sprockets::Railtie::ManifestNeededError when Sprockets is still present.

[brenogazzola]

Might be better to mention sprockets-rails too since #76 had a similar issue with it. I assumed a sprockets/webpacker project (which the guide is targeted at) would have sass-rails instead of any of the sprockets gems, but with more people raising the same point...

[sedubois]

Yes, the only reason sprockets became an explicit dependency in our project was to temporarily pin its version before later allowing an upgrade to version 4.

[stevegeek]

@sedubois Yeah something similar happened to me (just opened #81 with some other updates). Btw one suggestion here is maybe to change gemfile to Gemfile since I think it is mostly written capitalised.