pure-ftpd: fix hash and use .bz2 tarball

For some reason, the hash file added in commit
ee6c9f5a by Gustavo turns out to be
wrong, so this commit replaces it with the proper hash, also added
after checking the GPG signature.

While at it, we switch to using a .bz2 tarball.

Fixes:

  http://autobuild.buildroot.org/results/c44/c441f35119191f47dd5fae96fd76827024e50329/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>