Remove 'Uploaders' field from debian/control

`dpkg-buildpackage` complains if the maintainer field is the same as the
uploader field in `debian/control`:

    W: ruby-trollop source: maintainer-also-in-uploaders

Remove the 'Uploaders' field to resolve this.
Merge pull request #64 from alphagov/tidy-control-files

Remove 'Uploaders' field from debian/control