Pin to sprockets 3 as 4.0 support will require some work (fix tests with rails 4.2)

[jrafanie]

See: rails/sprockets-rails#443
and https://github.com/rails/sprockets/blob/master/UPGRADING.md#guide-to-upgrading-from-sprockets-3x-to-4

Note, config gem currently supports rails 4.2+ so it's unclear how easy it will
be to support sprockets 4 and rails 4.2. For now, we can use sprockets 3 and
have tests pass.

[cjlarose]

#250 is still open an includes the same change. Alternatively, #251 drops support for Rails 4.2 and would obviate the need for this change.

[jrafanie]

@cjlarose yeah, I'll leave this open and @pkuczynski can offer opinions:

• drop rails 4.2 would probably need a version bump (Drop support for Rails 4.2 #251), I can then close this PR
• Defer modification of ActionController::Base to when it is loaded #250 includes the same change as this PR so if that gets merged, I can close this one

I'm totally fine merging #250 and I can close this. Ultimately, I wanted to get #253 in and didn't want to include this change in it.

[pkuczynski]

I think the way it was done in #256 is better as it only affects Rails 4.2

[pkuczynski]

I think the way it was done in #256 is better as it only affects Rails 4.2