Follow Debian convention for naming the CKAN package

The convention for naming the Debian packages is:

```
<foo>_<VersionNumber>-<DebianRevisionNumber>_<DebianArchitecture>.deb
```
We are currently naming it as such in the docs:

```
python-ckan-2.0_amd64.deb
```

It should be (notice the underscore):

```
python-ckan_2.0_amd64.deb
```

The necessary 301 redirect is already in place in the packaging server.