Bring rest-client_1_6_7 branch in sync with vendored code in ManageIQ/cfme repo

This commit corresponds with lib/gems/rest-client-1.6.7 at commit 839a4c6 in the ManageIQ/cfme source repo.

By way of explaining/documenting why we vendored this code in cfme,
from cfme commit b35c8aa5967e65dbd7192546f939e54fd2eee845 log:

Added RestClient to vendor/gems from master branch of https://github.com/SoapSeller/rest-client/commit/fed0962f95ad2e84bd849422637249df74659dda

This resolves: Allow setting of ssl version from https://github.com/archiloque/rest-client/pull/123

BugzID: 16321