CI: Dont install billiard/amqp/vine from git, as they are now "stable".

requirements/dev.txt

@@ -1,4 +1 @@
-https://github.com/celery/py-amqp/zipball/master
-https://github.com/celery/billiard/zipball/master
 https://github.com/celery/kombu/zipball/master
-https://github.com/celery/vine/zipball/master