Fix /etc/os-release

According to the [manual](http://www.freedesktop.org/software/systemd/man/os-release.html):
* `PRETTY_NAME` should include the release code name, if it exists;
* `VERSION_ID` is the machine-parsable version;
* `VERSION` is the human-parsable one and should include a code name, if it exists;
* `BUG_REPORT_URL` should point to the actual page a user is supposed to report a bug.