Lock Sprockets to v3.x

[spaghetticode]

Description

Ref #3374

This is a temporary fix that will allow master to be green again (so we can continue working on PRs) while we wait for rails/sprockets-rails#446 to be merged (or some other working solution).

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[cedum]

Just curious about CIRCLE_TEST_REPORTS var: why not using one of the built-in env flags: https://circleci.com/docs/2.0/env-vars/#built-in-environment-variables ? 🤔(CI or CIRCLECI, both booleans).

[spaghetticode]

I didn't know exactly what env variables were set in circleCi by default, so I went with one that's explicitly set in the file circle.yml

After looking at the list I think the best candidate is CI, wdyt?

[cedum]

Agree 👍 I usually see ENV['CI'] used in these cases

[kennyadsl]

@spaghetticode I'd lock it anyway (not on CI only), we have this failure even running tests locally and on extensions due to this issue.

[spaghetticode]

@kennyadsl locally one can use CI=true bundle exec rake, while (most of) extensions use circleCI already, so I guess the problem may be minimal. I was trying to avoid possible issues to people already on Sprockets 4, but maybe scenario actually does not exist 🤔
Also, consistency is a valuable thing, so I'm chaging this to lock the gem everywhere. If somebody shows some concerns then I can change it again before merging.

[kennyadsl]

Thanks, Andrea!

[kennyadsl]

@spaghetticode can you please rebase against upstream master to have Rails 5.1 specs out of the required checks?

[spaghetticode]

@kennyadsl it's already updated. I removed the requirements for Rails 5.1 branches by updating github configuration. PR is now ready to merge.