v0.3.4: Fix FFTW related warning on Julia 0.6 (#179)
Avoid `using FFTW` on Julia 0.6, as the FFTW package does not define a module there, leading a warning about failed precompilation.
Avoid `using FFTW` on Julia 0.6, as the FFTW package does not define a module there, leading a warning about failed precompilation.