Fix GCC 4.9 build errors

This fixes the build errors I'm getting with gcc 4.9.
```
pypa/parser/parser.cc:2621:66: error: converting to 'std::stack<pypa::TokenInfo>' from initializer list would use explicit constructor 'std::stack<_Tp, _Sequence>::stack(_Sequence&&) [with _Tp = pypa::TokenInfo; _Sequence = std::deque<pypa::TokenInfo>]'
```

Let me know if you prefer to have a constructor for ```State``` instead of this solution.

BTW: Thanks for writing the parser! :-)