Release version 0.8

Changes
-------

- Standard output is now being checked for errors when exiting to
  verify that e.g. the index has been completely written.

- Continuous integration has been updated from Trusty to Xenial.
  Another build job for Bionic has been added.

- A lot of warnings have been fixed in order to enable the
  continuous integration to build with `-Werror`.

- Tests have been refactored to avoid any use of pipes which may
  swallow error codes.

- An error has been fixed that caused `warn()` to call exit(3P),
  causing gob-fsck(1) to exit on the first warning.

- Added an editorconfig file.