[Observium] snmp.conf change for r7728 and up on Debian/Ubuntu

Hi all,

Due to a MIB move, if you svn up to the latest release, you might encounter silly output from the SNMP library in PHP.

This is the case if you have the php5-snmp module installed, and have added our mib directory to /etc/snmp/snmp.conf.

To fix this, replace: mibdirs /opt/observium/mibs by mibdirs /opt/observium/mibs:/opt/observium/mibs/net-snmp

I've updated the install docs to reflect this.

Tom