Proxy file generated on every request

[koekaverna]

Hello, first of all I would like to thank you for developing this bundle.

I found one bug in our production, because var/cache folder is readonly and writing is only available at warmup. I got error:

Cannot rename \"/tmp/AcsiomaticDeviceDetectorBundle__PM__DeviceDetectorDeviceDetecto16oePZ\" to \"/var/www/html/var/cache/prod/AcsiomaticDeviceDetectorBundle/AcsiomaticDeviceDetectorBundle__PM__DeviceDetectorDeviceDetectorGeneratedd1
e8956d6a5423635e0218acd8c636ae.php\": rename(/tmp/AcsiomaticDeviceDetectorBundle__PM__DeviceDetectorDeviceDetecto16oePZ,/var/www/html/var/cache/prod/AcsiomaticDeviceDetectorBundle/AcsiomaticDeviceDetectorBundle__PM__DeviceDetectorDeviceDetectorGeneratedd1e8956d6a5423635e0218acd8c636ae.php

I found the problem in return $factory->createProxy(...). It looking for class_exists before generating new class, but it's not loaded and there is no autoloading for this class.
I cheked how it works in DoctrineMongoDBBundle and found how they use it

I made the fix using same aproach:
PR: #36