Closes thecodecrate#67
There is a problem with city_lookup. It only loads the cities for the
first statue you called `cities` on. This is because `cities` memoizes
all the cities for the country, but `city_lookup` only loads the cities
for the state, and doesn't get called again for the other states.

Ex:

having the following `db/cities-lookup.yml`:

```yaml
BR:
  GO:
    "Mutunópolis": "Mutunópolis"
```

If I load first cities for other state, it does not include this missing
city:

```shell
bin/rails c
CS.cities(:sp, :br) # loading another state first
CS.cities(:go, :br).include?("Mutunópolis")

```

returned false but it should be true.

If I load this state first, it works correctly

```shell
bin/rails c
CS.cities(:go, :br).include?("Mutunópolis")

```

This commit changes `city_lookup` to load all the cities for the country.