Give Sightstone the Inspiration mastery as a passive.

For those who don't know/remember...

http://leagueoflegends.wikia.com/wiki/Inspiration_%28Season_Four_Mastery%29

It's a mastery that gives you exp while your around higher level allies. It was nice on supports because you tended to be unable to keep up with the rest of the team, specially if you don't build damage. However with the mastery changes, it is near impossible to put it anywhere without either gimping a support's ability to choose their mastery build, having it abused in other roles, or having it be so mediocre that no one will use it.

So my solution is to put it on the one item that every support (or at least should) build and the items it builds into. It would not only solve the problems of putting it as a mastery, it will also make a fair catch up mechanic for anyone who had a bad lane phase. (granted it would still probably be faster to just farm an open lane)

Examples:

Unique - I See What You Did There For every higher level ally champion that you are near you gain "X" exp every 10 seconds.

-or-

Unique - I See What You Did There If you are around an ally champion who is a higher level than you you gain "X" percent of how much exp you need to level up.