setup.py: make libbe._version optional.

Thanks to Timmie for pointing this out:

From: Timmie <timmichelsen@gmx-topmail.de>
Date: Thu, 25 Oct 2012 09:42:23 +0000 (UTC)
Subject: Re: [Be-devel] NotImplementedError with Bazaar

...
I notices the following glichtes during the install:

D:\be-be-master\be-be>python setup.py install
WARNING:root:unable to import libbe._version: No module named _version
Traceback (most recent call last):
  File "setup.py", line 6, in <module>
    from libbe import _version, version
ImportError: cannot import name _version
...

1 files changed, 4 insertions, 3 deletions

diff --git a/setup.py b/setup.py
index bd45d08..a8973de 100755
--- a/setup.py
+++ b/setup.py
@@ -3,10 +3,11 @@
 from distutils.core import setup
 import os.path
 
-from libbe import _version, version
+from libbe import version
 
-rev_id = _version.version_info["revision"]
-rev_date = _version.version_info["date"]
+
+rev_id = version.version_info['revision']
+rev_date = version.version_info['date']
 
 data_files = []