Unpin ffi and support ffi >= 0.13

[james-stocks]

We had to pin ffi to <= 0.12.2 due to changes in 0.13.
It's pinned in both InSpec dependencies and in our omnibus Gemfile.
This is storing up a dependency problem for the future - we should unpin and troubleshoot what's going wrong.
The issue that was raised for ffi (ffi/ffi#784) suggests that there is no bug in ffi itself, but errors are now manifesting in other libraries that aren't using ffi 100% correctly (e.g. freeing memory, using fully correct function parameters) but have gotten away with it to-date.

[james-stocks]

Transferred this issue from inspec/inspec - train is where we need to troubleshoot the crash.

[tas50]

Fixed in f6055c9