Incorrect Hessian by `exp` function

[rzyu45]

Incorrect Hessian by `exp` function

[mcabbott]

This is probably fixed by #481, as master gives the right answer:

julia> H([1,0]) ≈ H([1,1e-10])
true

(jl_JRlWH7) pkg> st ForwardDiff
Status `/private/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_JRlWH7/Project.toml`
  [f6369f11] ForwardDiff v0.11.0-DEV `https://github.com/JuliaDiff/ForwardDiff.jl.git#master`

(#481 was pulled from v0.10 as it seemed that some people relied on the old behaviour, and we haven't tagged v0.11 yet... because making a breaking release of such a widely used package creates a lot of work downstream, and people have ideas for other things they would quite like to roll into a breaking 1.0 release.)