Upper-cased raw strings not supported

``` python
var = R"I'm a raw string"
```

```
E:\Temp>mypy upper_case_raw.py
upper_case_raw.py:1: error: Unterminated string literal
```

I believe this will be solved by the updated parser (https://github.com/python/mypy/issues/1353) but couldn't verify since I was unable to install the `typed-ast` pypi module on my Windows machine (no binary wheels and couldn't compile).
