fix multiple captcha on page

Hello!

This pr fix error when we have multiple captcha on single page.
I'll try to explain:
CaptchaType is defined as service - when we have multiple captcha on page eg. captcha1, captcha2, captcha3 key variable (https://github.com/Gregwar/CaptchaBundle/blob/master/Type/CaptchaType.php#L33) is:
after first captcha generating - captcha1,
after second - captcha2,
after third - captcha3

But key i defined globally in CaptchaType class, not in buildView method so when symfony create view and generating url for image key alway targeting to last captcha.