Broken regexp in is_relative_path()

`textcode.strings.is_relative_path()` is completely broken:

```pycon
>>> from textcode.strings import is_relative_path
>>> is_relative_path("/usr")
Traceback (most recent call last):
File "", line 1, in
File "/home/jwilk/.local/lib/python2.7/site-packages/textcode/strings.py", line 200, in
relative = re.compile('^(?:([^/]|\.\.)[\w_\-]+/.*$', re.IGNORECASE).match
File "/usr/lib/python2.7/re.py", line 194, in compile
return _compile(pattern, flags)
File "/usr/lib/python2.7/re.py", line 251, in _compile
raise error, v # invalid expression
sre_constants.error: unbalanced parenthesis
```

Tested against git HEAD (bca0af4a6ff10f4ef0bef6459f2f06a17d21f9f6).

Found using [pydiatra](https://github.com/jwilk/pydiatra).