Let's not auto-require all DB adapters

Because we're using only one in a test run

gemfiles/active_record_41.gemfile

@@ -5,18 +5,18 @@ gem 'activerecord', '~> 4.1.0', require: 'active_record'
 
 platforms :ruby do
   if RUBY_VERSION > "2.1.0"
-    gem 'sqlite3'
+    gem 'sqlite3', require: false
   else
-    gem 'sqlite3', '1.3.8'
+    gem 'sqlite3', '1.3.8', require: false
     gem 'nokogiri', '~> 1.6.8'
   end
-  gem 'pg'
-  gem 'mysql2'
+  gem 'pg', require: false
+  gem 'mysql2', require: false
 end
 platforms :jruby do
-  gem 'activerecord-jdbcsqlite3-adapter'
-  gem 'activerecord-jdbcpostgresql-adapter'
-  gem 'activerecord-jdbcmysql-adapter'
+  gem 'activerecord-jdbcsqlite3-adapter', require: false
+  gem 'activerecord-jdbcpostgresql-adapter', require: false
+  gem 'activerecord-jdbcmysql-adapter', require: false
 end
 platforms :rbx do
   gem 'rubysl', '~> 2.0'

gemfiles/active_record_42.gemfile

@@ -5,18 +5,18 @@ gem 'activerecord', '~> 4.2.0', require: 'active_record'
 
 platforms :ruby do
   if RUBY_VERSION > "2.1.0"
-    gem 'sqlite3'
+    gem 'sqlite3', require: false
   else
-    gem 'sqlite3', '1.3.8'
+    gem 'sqlite3', '1.3.8', require: false
     gem 'nokogiri', '~> 1.6.8'
   end
-  gem 'pg'
-  gem 'mysql2'
+  gem 'pg', require: false
+  gem 'mysql2', require: false
 end
 platforms :jruby do
-  gem 'activerecord-jdbcsqlite3-adapter'
-  gem 'activerecord-jdbcpostgresql-adapter'
-  gem 'activerecord-jdbcmysql-adapter'
+  gem 'activerecord-jdbcsqlite3-adapter', require: false
+  gem 'activerecord-jdbcpostgresql-adapter', require: false
+  gem 'activerecord-jdbcmysql-adapter', require: false
 end
 platforms :rbx do
   gem 'rubysl', '~> 2.0'

gemfiles/active_record_50.gemfile

@@ -5,14 +5,14 @@ gem 'activerecord', '~> 5.0.0', require: 'active_record'
 gem 'actionview', '~> 5.0.0', require: 'action_view'
 
 platforms :ruby do
-  gem 'sqlite3'
-  gem 'pg'
-  gem 'mysql2'
+  gem 'sqlite3', require: false
+  gem 'pg', require: false
+  gem 'mysql2', require: false
 end
 platforms :jruby do
-  gem 'activerecord-jdbcsqlite3-adapter'
-  gem 'activerecord-jdbcpostgresql-adapter'
-  gem 'activerecord-jdbcmysql-adapter'
+  gem 'activerecord-jdbcsqlite3-adapter', require: false
+  gem 'activerecord-jdbcpostgresql-adapter', require: false
+  gem 'activerecord-jdbcmysql-adapter', require: false
 end
 platforms :rbx do
   gem 'rubysl', '~> 2.0'

gemfiles/active_record_51.gemfile

@@ -5,14 +5,14 @@ gem 'activerecord', '~> 5.1.0', require: 'active_record'
 gem 'actionview', '~> 5.1.0', require: 'action_view'
 
 platforms :ruby do
-  gem 'sqlite3'
-  gem 'pg'
-  gem 'mysql2'
+  gem 'sqlite3', require: false
+  gem 'pg', require: false
+  gem 'mysql2', require: false
 end
 platforms :jruby do
-  gem 'activerecord-jdbcsqlite3-adapter'
-  gem 'activerecord-jdbcpostgresql-adapter'
-  gem 'activerecord-jdbcmysql-adapter'
+  gem 'activerecord-jdbcsqlite3-adapter', require: false
+  gem 'activerecord-jdbcpostgresql-adapter', require: false
+  gem 'activerecord-jdbcmysql-adapter', require: false
 end
 platforms :rbx do
   gem 'rubysl', '~> 2.0'