Update os.symlink skeleton to Python 3.4

This is a pretty trivial change - I'm using os.symlink and it's annoying in PyCharm when it complains about my using the `target_is_directory` argument.

It looks like there are a few inconsistencies between the Python 3.4 docs for `os` and the skeletons but I don't have time right now to fix them. Official docs here: https://docs.python.org/3.4/library/os.html#os.symlink

Note that I don't know how to represent the signature:
```python
os.symlink(source, link_name, target_is_directory=False, *, dir_fd=None)
```
and keep Python 2 compatibility so I represented:
```python
os.symlink(source, link_name, target_is_directory=False, dir_fd=None)
```
instead.