Make DB configuration RoR 5.1 compatible.

The configuration does not accept symbols as a keys anymore, since
rails/rails@0a4f600,
so the keys has to be strings.

Alternatively, HashWithIndiferentAccess could be used instead of plain
Hash.

However, it is not clear if this RoR change was intentional.

Author: voxik

test/helper.rb

@@ -30,7 +30,7 @@ class ActiveSupport::TestCase
   ActiveRecord::Base.raise_in_transactional_callbacks = true
 end
 
-ActiveRecord::Base.configurations = { "test" => { adapter: 'sqlite3', database: ':memory:' } }
+ActiveRecord::Base.configurations = { 'test' => { 'adapter' => 'sqlite3', 'database' => ':memory:' } }
 ActiveRecord::Base.establish_connection(:test)
 
 ActiveRecord::Schema.verbose = false