Deprecation warning due to invalid escape sequences in Python 3.7

Deprecation warning are raised due to invalid escape sequences. This can be fixed by using raw string or escaping them.

```
find . -iname '*.py'  | xargs -P 4 -I{} python -Walways -m py_compile {}

./geocoder/uscensus.py:36: DeprecationWarning: invalid escape sequence \d
  match = re.search('^\d+', self.address, re.UNICODE)
```