Bump Graph version to v2.6

[SammyK]

Graph v2.6 is the new hotness. :)

[yguedidi]

@SammyK you know this is a BC break in fact? ;)

[SammyK]

Yes, for those who are using the default fall-back version of Graph. That's why I wanted the next version of the SDK to throw an exception if you don't provide a default Graph version. But in the docs all the examples have the Graph version in the options array so for the copy-pasters, they'll have their Graph version set.

So yeah, it's BC for a very small subset, but I feel like we decided to keep bumping the Graph version every time a new one came out. But I'm cool with just keeping v2.5 for the default fallback in Facebook and just updating the docs to v2.6. Then the developers who are using the fall-back option will get upgraded to the next version of Graph when v2.5 is force-upgraded on April 12, 2018.

What do you guys think?

[yguedidi]

That's why I wanted the next version of the SDK to throw an exception if you don't provide a default Graph version.

Me too!

I feel like we decided to keep bumping the Graph version every time a new one came out.

You're right, I'm not really against this change as we used to do it in the 5.x major version. I said it more like a reminder for 6.x

[gfosco]

@SammyK @yguedidi You're now core collaborators with write privileges. Wield your powers responsibly and for great justice. 🎉

[SammyK]

You the man @gfosco! :)