Remove String 1.8.7 -> 1.9.3 compatibility crutches.

String#+(exception) was removed in 1.9, we should be calling String#+(exception.message).
Let it raise errors so we fix any remaining callers.

Fixes #536
Remove 1.8.7 -> 1.9.3 compatibility crutch.

String#ord exists on 1.9+ and since we have no remaining 1.8.7 uses, it's no longer needed.

Fixes #536
We previously removed enumerable from String so this spec is not needed.

This was missed in commit fe3833c69e1d.  :bomb:
Merge pull request #573 from jrafanie/remove_1_9_crutches

Remove String 1.8.7 -> 1.9.3 compatibility crutches.