Fix problem with README.md inclusion

This meant I couldn't install from PyPI (log below).

I think `package_data` has to refer to data in actual packages (i.e. `captainhook`).

```
% pip install captainhook
Downloading/unpacking captainhook
  Downloading captainhook-0.3.tar.gz
  Running setup.py egg_info for package captainhook
    Traceback (most recent call last):
      File "<string>", line 14, in <module>
      File "/home/daniel/.virtualenvs/ott/build/captainhook/setup.py", line 4, in <module>
        long_description = open('README.md').read()
    IOError: [Errno 2] No such file or directory: 'README.md'
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 14, in <module>

  File "/home/daniel/.virtualenvs/ott/build/captainhook/setup.py", line 4, in <module>

    long_description = open('README.md').read()

IOError: [Errno 2] No such file or directory: 'README.md'

----------------------------------------
Command python setup.py egg_info failed with error code 1 in /home/daniel/.virtualenvs/ott/build/captainhook
```