gnu: emacs-use-package: Update to commit da8c9e2.

This update is needed because the release tarball from github does not
contain all the files needed for us to build the package.

* gnu/packages/emacs.scm (emacs-use-package): Update to commit da8c9e2.
[source]: change to git.
[arguments]: Enable tests.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>