Lock sprockets to 3.x on Ruby < 2.5

Sprockets 4.0 depend on Ruby >= 2.5 https://rubygems.org/gems/sprockets/versions/4.0.0

This results in build failures
(https://travis-ci.org/github/rspec/rspec-rails/jobs/713253057):

    Installing sprockets 4.0.2
    Gem::RuntimeRequirementNotMetError: sprockets requires Ruby version >= 2.5.0.
    The current ruby version is 2.4.10.364.

This is a forgotten forward-port from https://github.com/rspec/rspec-rails/pull/2360/files#diff-8c3955be04b733dbd31e54e50844fb84R31

Surprisingly, Ruby 2.3.8 and Ruby 2.2.10 builds don't fail, and install
sprockets 3.7.2.

Author: pirj

Gemfile-rails-dependencies

@@ -18,6 +18,7 @@ when /stable$/
   gem_list = %w[rails railties actionmailer actionpack activerecord activesupport activejob actionview]
   gem 'puma', "3.12.1" if version > '5-0-stable'
   gem 'activerecord-jdbcsqlite3-adapter', git: 'https://github.com/jruby/activerecord-jdbc-adapter', platforms: [:jruby]
+  gem "sprockets", '~> 3.0' if RUBY_VERSION < '2.5'
 
   gem_list.each do |rails_gem|
     gem rails_gem, :git => "https://github.com/rails/rails.git", :branch => version
@@ -28,7 +29,7 @@ when nil, false, ""
   gem 'activerecord-jdbcsqlite3-adapter', platforms: [:jruby]
 else
   gem "rails", version
-
+  gem "sprockets", '~> 3.0' if RUBY_VERSION < '2.5'
   gem "puma" if version >= '5-1-stable'
 
   if version.gsub(/[^\d\.]/,'').to_f >= 6.0