Add "airport" to the PlaceAutocompleteType enum

Hello, thanks for your work, I like your library a lot.... I would like to use the `PlacesApi.placeAutocomplete` for getting airports.

From my local tests simply adding `airport` to the `PlaceAutocompleteType` enum would do the trick
`
public enum PlaceAutocompleteType implements StringJoin.UrlValue {
GEOCODE("geocode"),
ADDRESS("address"),
AIRPORT("airport"), // <-- here
ESTABLISHMENT("establishment"),
REGIONS("(regions)"),
CITIES("(cities)");

PlaceAutocompleteType(final String placeType) {
this.placeType = placeType;
}

private final String placeType;

@Override
public String toUrlValue() {
return placeType;
}

@Override
public String toString() {
return placeType;
}
}
`

Many thanks
Stephane