`exp_range` doesn't hit powers of 2

[stevenjswanson]

>>> list(exp_range(2,128,2**0.25))
[2, 3, 4, 5, 6, 7, 9, 11, 13, 15, 19, 22, 26, 31, 38, 45, 53, 63, 76, 90, 107, 127, 152]

But we really should hit the powers of 2. We miss 8, 16, 32, etc.