power_rev incorrect for odd powers

[dpsanders]

julia> b
[-1.00001e+16, 1]

julia> x
[-1e+08, 1e+08]

julia> IntervalConstraintProgramming.power_rev(b, x, 3)
([-1.00001e+16, 1],[0, 1],[3, 3])

[dpsanders]

Simpler example:

julia> b = -27..27
[-27, 27]

julia> x = -5..5
[-5, 5]

julia> IntervalConstraintProgramming.power_rev(b, x, 3)
([-27, 27],[0, 3.00001],[3, 3])

Here, x should end up as [-3,3].

[dpsanders]

Fixed in #49