Installation issue: Exception: Cython-generated file 'pandas/index.c' not found
I'm attempting to install pandas via pip on Ubuntu 14.04 (inside Docker, for what it's worth, so doing it as root with no virtualenv - the Docker container serves that purpose.). I get the following exception:

```
    Exception: Cython-generated file 'pandas/index.c' not found.
                    Cython is required to compile pandas from a development branch.
                    Please install Cython or download a release package of pandas.
```

This is strange, because earlier this week the exact same configuration built correctly. What's changed? Is there something else I ned to do?
