switch order of method/object in Future::then to match std::bind

switch order of method/object in Future::then to match std::bind

Summary:
I have half a mind to just rip this out and let people use std::bind if they need this. But I won't be so cruel.

Why isn't this just implemented as `then(std::bind(method, object))` anyway? Is the template soup we have now faster?

Test Plan:
Fixed the unit tests to use the new format.
Will look to contbuild to catch all the things this might break (if anyone is using it at all?), and will fix them.

Reviewed By: hannesr@fb.com

Subscribers: trunkagent, exa, folly-diffs@, yfeldblum, jsedgwick, davejwatson

FB internal diff: D1831118

Signature: t1:1831118:1423243771:65db9a89daf14d8bd88331c503ba1ea7ab03b679