Fix deb version parsers (and libav package).

The current versions of e.g. libavcodec-dev in Debian unstable look
like this: "6:11-1". Fix the version parsers to handle that.

This fixes the deb install/check for the libav recipe.
Merge pull request #109 from uwehermann/deb_version_and_libav_fix

Fix deb version parsers (and libav package).