Gemfile make sure deps are read from gemspec

See discussion on https://github.com/thisMagpie/fftw/issues/19

## Problem:
Gemfile instructs bundle to use 0.9 but gemspec instructs use of nmatrix > 0.9.

## Solution:
Gemfile make sure deps are read from gemspec so  nmatrix > 0.9 is used instead.