pwn.ssh fails if ~/.ssh/config doesn't exist

[Eckankar]

Traceback (most recent call last):
  File "./assignment11.py", line 11, in <module>
    s = ssh('server:port', 'user', 'pass')
  File "/home/sebbe/build/pwntools/lib/pwn/ssh.py", line 195, in __init__
    self._connect()
  File "/home/sebbe/build/pwntools/lib/pwn/ssh.py", line 234, in _connect
    conf.parse(open(os.path.expanduser('~/.ssh/config')))
IOError: [Errno 2] No such file or directory: '/home/sebbe/.ssh/config'

[TethysSvensson]

Fixed in #5