compose: deploy.replicas doesn't work with environment variables

Description

services.service.deploy.replicas in Compose file version 3 with Docker Compose 1.10-rc2 doesn’t seem to work with environment variables. If a variable instead of a literal integer is used, then Docker Compose says it contains an invalid type

Steps to reproduce the issue:

1. Create a compose file where the value for replicas is a variable instead of a literal integer, like the following.

version: '3'
services:
  node-chrome:
    image: selenium/node-chrome:3.0.1
    deploy:
      replicas: $NUM_NODES_CHROME

1. Define a value for NUM_NODES_CHROME. In my case, I declared NUM_NODES_CHROME=2 in a .env file.

Describe the results you received:

Docker Compose says that replicas contains an invalid type.

ERROR: The Compose file './selenium.yml' is invalid because:
services.node-chrome.deploy.replicas contains an invalid type, it should be an integer

Describe the results you expected:

Docker Compose should substitute $NUM_NODES_CHROME with 2, treating the value as an integer.

Output of docker-compose --version:

docker-compose version 1.10.0-rc2, build fb241d0