Blacklist DES from cipher suite selection

Per the comments in cmd.py:

Instate a cipher suite that bans 3DES, which has been seen to be
selected ahead of RC4, S3's preference at time of writing, on Ubuntu
Precise.  It is incredibly slow and consumes a lot of resources when
encrypting the amount of data that WAL-E regularly sends.

Only Python 2.7+ possesses the 'ciphers' keyword to wrap_socket.

On Ubuntu Raring RC4 is preferred without this, but as use of
Precise is very real in production for years to come, acquiesce and
ban DES everywhere.