Update requirements for PySNMP
Update requirements for PySNMP

Otherwise `twistd -n tensor -c <my_config.yml>` fails if there are SNMP checks therein:

```
<snip>
File "/usr/local/lib/python2.7/dist-packages/tensor/sources/snmp.py", line 20, in <module>
    from pysnmp.entity.rfc3413.twisted import cmdgen
ImportError: No module named twisted
``
hobbeswalsh/tensor,calston/tensor,hobbeswalsh/tensor,calston/tensor,calston/tensor,calston/tensor