system-source: Bump the version in system-expand

Bump the version in system-expand, so new 3.7 features will be included
in the expanded system() when running the program.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #212 from algernon/f/system-expand-3.7

system-source: Bump the version in system-expand