Change file mode from `rwb` to `r+b`

In `wal_e/worker/worker_util.py:29` and `wal_e/worker/upload.py:74` you
use `rwb` as file mode. If you look at the Python documentation for
[Reading and Writing files] you can see that the only possible options
are:

- `w` for write only
- `r` for read only
- `r+` for read and write

You can also always append `b` to open the file in binary mode. So I
think that in case of wal-e this should be `r+b` instead of `rwb`. I made
this change in the hope of fixing an error I got on FreeBSD.

[Reading and Writing Files]: http://docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files