Add lipbostal servic

The Pelias API has now been modified to expect libpostal to live as a
separate HTTP service rather than being included in memory.

This PR adds the libpostal service to `docker-compose.yml` but doesn't
change anything else. The current (as of 2018/3/19) `production` version
of Pelias API does not yet use the libpostal service. It's convenient to
have this merged so that we can start the process of switching over,
though.

Connects #42

docker-compose.yml

@@ -32,6 +32,12 @@ services:
     volumes:
       - "./pelias.json:/code/pelias.json"
       - "libpostaldata:/usr/share/libpostal"
+  libpostal:
+    image: pelias/go-whosonfirst-libpostal
+    container_name: pelias_libpostal
+    restart: always
+    ports: [ "8080:8080" ]
+    networks: [ "pelias" ]
   placeholder:
     depends_on: [ "baseimage" ]
     image: pelias/placeholder