pin python3-digest to latest version in requirements

If one were to follow the "Running the Tests" docs with a current version of pip, the requirements file will fail to install all packages, as pip>=1.4 no longer installs prerelease packages by default.

Instead of globally enabling this by modifying the docs to add a `--pre` flag, which is too big of a hammer and also doesn't exist on pip 1.3, specifying that the latest beta version of python3-digest is acceptable allows us to waive the behavior on just that package. This should preserve behavior for existing cases and fix it for those on the latest versions.