Parse fail for Mapbox vector_tile.proto

Running Falanx on the Mapbox [vector_tile.proto](https://github.com/mapbox/vector-tile-spec/tree/master/2.1/vector_tile.proto) fails with:

```
Generating code for: vector_tile.proto
ERROR: proto file was not able to be parsed.

Error in Ln: 21 Col: 23
optional string string_value = 1;
^
Expecting: whitespace
```

I have spent some time trying to figure out what causes it, to no avail. The only way I can get falanx to process the file, is by removing _all_ field specifications, leaving only the empty messages.