Symfony - Class Omnipay\PayPal\ExpressGateway not found

Question

I'm trying to integrate the Omnipay Paypal package with my Symfony2.5 application. I've installed the colinodell/omnipay-bundle package, and followed the instructions on how to set it up.

Set config as:

omnipay:
    methods:
        PayPal_Express:
            username: ******
            password: *******
            signature: ********
            testMode: false
            solutionType: Sole
            landingPage:  Login
    default_gateway: PayPal_Express

and from the controller :

$paypal = $this->get('omnipay')->getDefaultGateway();
$formData = ['number' => '4242424242424242', 'expiryMonth' => '11','expiryYear' => '2018', 'cvv' => '123'];
$response = $paypal->purchase($formData)->send();

I'm getting the error

Class '\Omnipay\Paypal\ExpressGateway' not found"

Is there something I'm forgetting?