Update to the latest on github:
    
    - Add support for more kinds of function redefinition
    - Allow conditionally assigning None to a module
    - Support conditionally defined nested functions
    - Tighten argument type for Instance(erased=...) from Any to bool.
    - Reformat a few messages so they are easier to find using grep.
    - Update README.md to fix installation instructions for Python 3.5