Bump futures to 2.1.6 for swiftclient

Partial-Bug: #1361931

Conflicts:
	global-requirements.txt

Change-Id: I3161f5bd552b600d2a4483630bed6697e87433e9
(cherry-picked from commit d86823000b67236812a6a5596a594b5422370203)
(cherry picked from commit c99cbdc7d3a1e5f5b29b9f1507fad2e62e09485c)