Drop support for boost below 1.61
Drop support for boost below 1.61

Summary: [Folly] Drop support for boost below 1.61, which means removing multiple versions of support for `boost-context` embedded in `folly/fibers/`.

Reviewed By: fredemmott

Differential Revision: D22779795

fbshipit-source-id: fafc9b38784dcb206283e11d01d77dd8655354a6

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly