Fix factory girl dependency for Solidus < 2.5

We need to load a factory_bot version that has factory_girl in it
to support Solidus versions < 2.5

This change also includes conditional logic for the database
interface gems.

Gemfile

@@ -3,14 +3,26 @@ source 'https://rubygems.org'
 branch = ENV.fetch('SOLIDUS_BRANCH', 'master')
 gem 'solidus', github: 'solidusio/solidus', branch: branch
 
-gem 'rails-controller-testing', group: :test
+group :test do
+  gem 'rails-controller-testing'
+  if branch < "v2.5"
+    gem 'factory_bot', '4.10.0'
+  else
+    gem 'factory_bot', '> 4.10.0'
+  end
+end
+
 if branch == 'master' || branch >= "v2.3"
   gem 'rails', '~> 5.1.0' # HACK: broken bundler dependency resolution
 elsif branch >= "v2.0"
   gem 'rails', '~> 5.0.0' # HACK: broken bundler dependency resolution
 end
-gem 'pg', '~> 0.21'
-gem 'mysql2', '~> 0.4.10'
+
+if ENV['DB'] == 'mysql'
+  gem 'mysql2', '~> 0.4.10'
+else
+  gem 'pg', '~> 0.21'
+end
 
 # Provides basic authentication functionality for testing parts of your engine
 gem 'solidus_auth_devise', '~> 1.0'