swc-windows-installer.py: Use POSIX paths for ~/nano.rc

On Wed, Mar 12, 2014 at 06:34:43PM -0700, Ethan White wrote [1]:
> The bash script in #228 is now working properly, but the Python
> script is still failing. It reports errors like:
>
>     Error reading ~\.swc\share\nanorc\doc\syntax\python.nanorc
>
> The obvious difference is that the nano.rc files produced by the
> bash script have `/` for paths for the includes, but the Python
> script produces includes that use `\`.

[1]: https://github.com/swcarpentry/bc/pull/357#issuecomment-37491038
Merge branch 'python' into bc-namespaced-python

* python:
  swc-windows-installer.py: Use POSIX paths for ~/nano.rc