Launch agent loading question
All jobs related to items installed by packages (i.e. a pkg installer) are done with preflight or postflight scripts which are included in the package. 

A preflight script may check if a needed folder exists and if its permissions are set properly. A postflight script may load launch agents and daemons installed by the pkg installer and set their owners and permissions.

In your case you would use a postflight script like this one:

#!/bin/bash

sudo /bin/launchctl load /Library/LaunchAgents/net.company_name.blocksierra.plist




BTW: Using if [[ $Version -ge 11 ]] will unload and delete the launch agent when El Capitan is installed. You may have to change this to 12.