AttributeError: 'SymbolIndex' object has no attribute 'get_or_create_index'

Attempting to either run the example code in the README:
```
index = importmagic.SymbolIndex()
index.get_or_create_index(name='foo', paths=sys.path)
```
— or attempting to run the same code as it's written in the `bin/importmagic` script results in the same error in both cases:
```
Traceback (most recent call last):
File "/home/brhodes/imagic", line 33, in
index.get_or_create_index(paths=path, refresh=args.refresh)
AttributeError: 'SymbolIndex' object has no attribute 'get_or_create_index'
```
I double-checked whether I have the library installed correctly and haven't yet found any problems with my install. Thanks for any hints in getting the library working!