Don't use :math.pow in elixir grains, fix parens
Don't use :math.pow in elixir grains, fix parens

:math.pow(2, 64) == (:math.pow(2, 64) - 1) due to rounding issues,
this is not desirable.

Also move some parentheses to fix an error in the example.

tjcelaya/xelixir,parkerl/xelixir,victorlcampos/xelixir,montague/xelixir,elasticdog/xelixir,Egogre/xelixir,wobh/xelixir,exercism/xelixir,cezarykosko/xelixir,henrik/xelixir