no pg_hba.conf entry for replication connection from host

I have been configuring a slave server that needs to connect to the host. Both the master and the standby servers have a pg_hba.conf that looks like this:

    # Allow anyone to connect remotely so long as they have a valid username and

    # password.

    host    replication     ${REP_USER}     0.0.0.0/0               md5

    host    ${DB_NAME}      ${DB_USER}      0.0.0.0/0               md5

This should allow access from every IP address, right? Evidently, though, the standby server cannot connect using the REP_USER credentials via `primary_conninfo` 

    primary_conninfo = 'host=${MASTER_PORT_5432_TCP_ADDR} port=5432 user=${REP_USER} password=${REP_PASS}'

I know this doesn't work because I never see in my logs:

    LOG:  streaming replication successfully connected to primary

Instead, what I see is 

     LOG:  database system was interrupted; last known up at 2015-04-09 16:35:05 GMT

What am I doing wrong?