Fix Travis CI build
Fix Travis CI build

Summary:
Fix the following error:

```
fbthrift/thrift/compiler/generate/t_mstch_py3_generator.cc:376:59: error: cannot call member function 'bool {anonymous}::t_mstch_py3_generator::has_default_value(const apache::thrift::compiler::t_field&) const' without object
     return field->get_req() == t_field::e_req::T_REQUIRED ||
                                                           ^
```

Reviewed By: davidtgoldblatt

Differential Revision: D10178344

fbshipit-source-id: 050839ef34c56bb24a8bcf7384e8db6be4b31ba1

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift