remove obsolete md5 import routine

The old md5 module was deprecated since Python 2.5, and we even didn't
support that version already. The current implementation in
boto/utils.py will not even work under Python 2.4 because of the name
"md5" imported from md5 module was replaced by the md5 module in the
following routine, which will cause errors.

This PR just replaced the routine with a simple import from `hashlib`, which will work flawlessly in Python 2.5+ and 3.*.