Add VIRTUAL_ENV to docker-compose

This fixes `make test` in the docker-compose, as `up-reqs` checks for
this envvar before installing requirements.  Since this is in a docker
container for testing, just allow pip to install without a virtualenv.

Change-Id: I511efa9fab8d393bf9f2b8e80408fb8cf155bafa

docker-compose.yml

@@ -10,6 +10,7 @@ services:
     command: make test
     environment:
       SQLALCHEMY_DATABASE_URI: postgresql://lemur:lemur@postgres:5432/lemur
+      VIRTUAL_ENV: 'true'
 
   postgres:
     image: postgres:9.4