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.
Add warning about :math.pow to elixir grains
Merge pull request #1032 from pminten/elixir-grains-no-math-pow

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