avoid fileobj.flush() in decompressed_fileobj()

This avoid failures on BSD platforms where fileobj.flush()
fails on read-only files.  Instead the underlying file descriptor
is obtained via fileno() and reset to 0 via os.lseek()
Resolves issue #71
make sandbox.sh more compatible on BSD platforms

This simply removes the sed -r flag, as this was unnecessary and
failed platforms without GNU sed.
Resolves issue #70