Made attribute optional (as it is in pyOpenSSL)

See https://github.com/pyca/pyopenssl/commit/0d7e8a1af28ab22950b21afa3fd451cec7dd5fdc -- It looks like this constant isn't set on some platforms (including Raspberry Pi's libssl). Without this, mitmproxy just refuses to start:
```
(venv)pi@raspberrypi:~/proxies$ mitmproxy
Traceback (most recent call last):
  File "/usr/local/bin/mitmproxy", line 3, in <module>
    from libmproxy import proxy, console, cmdline
  File "/usr/local/lib/python2.7/dist-packages/libmproxy/proxy.py", line 4, in <module>
    from netlib import tcp, http, certutils, http_status, http_auth
  File "/usr/local/lib/python2.7/dist-packages/netlib/tcp.py", line 17, in <module>
    OP_MSIE_SSLV2_RSA_PADDING = SSL.OP_MSIE_SSLV2_RSA_PADDING
AttributeError: 'module' object has no attribute 'OP_MSIE_SSLV2_RSA_PADDING'
```

As far as I can tell, `OP_MSIE_SSLV2_RSA_PADDING` is never used in mitmproxy, so this shouldn't be a problem just leaving it unset