Fix Ruby 2.4 builds
Fix Ruby 2.4 builds

Ruby 2.4 builds are failing because the latest major version of Sprokets
is not installable on Ruby 2.4. We don't care about sprockets as a
dependency in Scenic, so I've pinned it to a compatible version for now.
When we stop testing on Ruby 2.4, we can remove this.

thoughtbot/scenic,thoughtbot/scenic,thoughtbot/scenic