Add missing dependencies for Debian-based installation instructions

I had some difficulties installing pdf-redact-tools on my Mint machine because I was missing the [python-stdeb](https://packages.debian.org/wheezy/python-stdeb) and [python-all](https://packages.debian.org/wheezy/python-all) packages. This Pull Request updates the Debian-based installation instructions to include these packages.

This might also fix the problem mentioned in issue #4