Unpin Sprockets gem and upgrade to version 4.

We had this pinned to version 3.7.2 because of a segmentation
fault that occurred when upgrading to 4.0+

After investigation it seems that Sprockets now uses the manifest.js
file to compile assets instead of the assets.rb file.

See: sass/sassc-rails#122 and
https://github.com/rails/sprockets/blob/master/UPGRADING.md#manifestjs

This commit upgrades Sprockets and now uses the manifest.js file
to precompile assets which ensures the segmentation fault no
longer occurs.

Gemfile

@@ -25,7 +25,7 @@ gem "rqrcode"
 gem "sassc-rails"
 gem "sentry-rails"
 gem "sentry-ruby"
-gem "sprockets", "3.7.2"
+gem "sprockets"
 gem "two_factor_authentication"
 gem "tzinfo-data"
 gem "uk_postcode", "~> 2.1"

Gemfile.lock

@@ -409,7 +409,7 @@ GEM
     spring (2.1.1)
     spring-commands-rspec (1.0.4)
       spring (>= 0.9.1)
-    sprockets (3.7.2)
+    sprockets (4.0.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.2.2)
@@ -499,7 +499,7 @@ DEPENDENCIES
   simplecov
   simplecov-console
   spring-commands-rspec
-  sprockets (= 3.7.2)
+  sprockets
   timecop (~> 0.9.2)
   two_factor_authentication
   tzinfo-data

app/assets/config/manifest.js

@@ -1,3 +1,7 @@
 //= link_tree ../images
 //= link_directory ../stylesheets .css
 //= link_directory ../javascripts .js
+//= link accessible-autocomplete.min.css
+//= link html5shiv.js
+//= link govuk-logotype-crown.png
+//= link accessible-autocomplete.min.css

config/initializers/assets.rb

@@ -3,20 +3,11 @@
 # Version of your assets, change this if you want to expire all your assets.
 Rails.application.config.assets.version = "1.0"
 
-Rails.application.config.assets.precompile += %w[
-  govuk-logotype-crown.png
-  application_ie8.css
-  html5shiv.js
-  accessible-autocomplete.min.js
-  accessible-autocomplete.min.css
-  tick.svg
-  cross.svg
-]
-
 Rails.application.config.assets.paths += [
   Rails.root.join("node_modules/govuk-frontend/govuk/assets/images"),
   Rails.root.join("node_modules/govuk-frontend/govuk/assets/fonts"),
   Rails.root.join("node_modules/govuk-frontend/govuk"),
   Rails.root.join("node_modules/govwifi-shared-frontend/dist"),
   Rails.root.join("node_modules/html5shiv/dist"),
+  Rails.root.join("vendor/assets/stylesheets"),
 ]