bipython can't find ipython dependency

[maxrothman]

I installed bipython via pip on OSX. When trying to start it, I get the following error:

Traceback (most recent call last):
  File "/usr/local/bin/bipython", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 2603, in <module>
    working_set.require(__requires__)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 666, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 565, in resolve
    raise DistributionNotFound(req)  # XXX put more info here
pkg_resources.DistributionNotFound: ipython>=1.0

I definitely have ipython installed:

$ pip freeze
...
ipython==3.1.0
...