version-helper: Also try parsing release date from Cargo.toml
The package.metadata.gstreamer.release_date
date string can be used to
specify the release date.
This is used if there's no git repository as a fallback before using the mtime of the Cargo.toml. Using the mtime will fail when building a crate packaged by cargo because cargo sets the mtimes to bogus values.
Fixes #440 (closed)