Upgrade ruby and rails (#868)

https://eaflood.atlassian.net/browse/RUBY-1061

This PR contails all the changes necessary to upgrade the Ruby version from 2.4 to 2.7.1 and Rails version from 4.2 to 6.0.3.1

Some notes about the updates:
- Request tests syntax for params and headers to send along with a `get`, `put` or `post` request have changed. Now the params are into a key-value format with the key specifying what are `params` and what are `headers`
- With Rails 6, the `rake` command is not invoked directly any more. Instead, we now use `bin/rails`.
- Since Rails 6 was built with the. intent to leave the assets work to. NodeJs, we had to require the `sassc-rails` gem on top of the rest of the gems in order for the old way of dealing with assets still works.
- `POST` request that do not specify a render will redirected to a `GET` automatically, hence we have to explicitly raise errors now if we want them not to be reachable
- Inherithance and scoping are something we must make more attention too. We used to define stuff like:
```
module WasteCarriersEngine
  class ErrorsController < ApplicationController
```
and trust that ruby would resolve the `ApplicationController` to be the module-scoped definition of it. We can't trust that any more, hence we have to be specific and do stuff like:
```
module WasteCarriersEngine
  class ErrorsController < ::WasteCarriersEngine::ApplicationController
```
- Parameters on controller coming from a request are not any more instances of the Hash class, but instances of the `ActionController::Parameters` class
- The test suite will run in `:random` mode by default
- Many of the spec matches we were used to in request tests have been externalised to a gem `rails-controller-testing`

.ruby-version

@@ -1 +1 @@
-2.4.2
+2.7.1

.travis.yml

@@ -5,7 +5,7 @@ addons:
     organization: "defra"
 
 language: ruby
-rvm: 2.4.2
+rvm: 2.7.1
 cache: bundler
 
 # Travis CI uses shallow clone to speed up build times, but a truncated SCM
@@ -40,7 +40,7 @@ services:
 
 before_install:
   - export TZ=UTC
-  - gem install -v 1.17.3 bundler
+  - gem install bundler
   - sudo apt-get install xvfb -y
   - sudo apt-get install ttf-liberation -y
   - sudo apt-get install wkhtmltopdf -y

Gemfile

@@ -1,7 +1,11 @@
 # frozen_string_literal: true
 
 source "https://rubygems.org"
-ruby "2.4.2"
+ruby "2.7.1"
+
+# Temporary workaround until we implement webpack assets
+# See: https://github.com/sass/sassc-rails/issues/114
+gem "sassc-rails"
 
 # Declare your gem's dependencies in waste_carriers_engine.gemspec.
 # Bundler will treat runtime dependencies like base dependencies, and
@@ -16,7 +20,7 @@ gem "govuk_template", "~> 0.23"
 gem "jquery-rails"
 
 # Use MongoDB as the database
-gem "mongoid", "~> 5.2"
+gem "mongoid"
 # Implement document-level locking
 gem "mongoid-locker"
 
@@ -38,12 +42,12 @@ group :development, :test do
   # Apply our style guide to ensure consistency in how the code is written
   gem "defra_ruby_style"
   gem "dotenv-rails"
-  gem "rspec-rails", "~> 3.6"
+  gem "rspec-rails"
 end
 
 group :development do
   # Access an IRB console on exception pages or by using <%= console %> in views
-  gem "web-console", "~> 2.0"
+  gem "web-console"
 
   # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
   gem "spring"
@@ -57,6 +61,7 @@ end
 group :test do
   gem "database_cleaner"
   gem "factory_bot_rails", require: false
+  gem "rails-controller-testing"
   gem "simplecov", "~> 0.17.1", require: false
   gem "timecop"
   gem "webmock", "~> 3.4"