Change FactoryGirl::Generators::Base#explicit_class_option to output 1.9 Hashes

This changes the generator to use the 1.9-style Hash syntax for the `class` option. So, where previously the generator would output:

```ruby
FactoryGirl.define do
  factory :buttery_biscuit_base, :class => 'Buttery::BiscuitBase' do
    #...
  end
end
```

it will now output:

```ruby
FactoryGirl.define do
  factory :buttery_biscuit_base, class: 'Buttery::BiscuitBase' do
    #...
  end
end
```

I totally understand this is a very minor change and is ultimately down to personal preference. FactoryGirl requirs Ruby > 1.9 (https://github.com/thoughtbot/factory_girl/blob/master/factory_girl.gemspec#L17) so this shouldn't cause any issues in practice.