(maint) Remove GC bug workaround for Ruby < 1.8.7

Commit bd5dc649 added a workaround for a Ruby GC bug that was present in
1.8.5, 1.8.6, and partially fixed but still residual in 1.8.7. Since
we're dropping Ruby 1.8 support entirely we can now remove the
workarounds.
Merge pull request #3094 from adrienthebo/removal/master/remove-187-gc-bug-workaround

(maint) Remove GC bug workaround for Ruby < 1.8.7