Release version 0.6

Changes
-------

- Build instructions and tests have been adjusted to not include
  non-POSIX instructions. This enables support for OpenBSD.

- When writing a block into the block store, gob will now first
  write everything into a temporary file and then rename that
  file to have the final name. This will ensure that gob will
  never put partially written blocks at a final block path.

- Travis CI has been fixed to check the signature of packages
  installed via Bintray.