Remove warning about missing nix/sources.nix

[basvandijk]

When fetching sources.nix from niv itself using:

builtins.fetchurl
  https://raw.githubusercontent.com/nmattia/niv/506b896788d9705899592a303de95d8819504c55/nix/sources.nix;

it would be nice if the following error is removed from the niv` tool:

  WARNING: Could not read nix/sources.nix
     ( nix/sources.nix: openBinaryFile: does not exist (No such file or directory) )

[nmattia]

Hey @basvandijk, would love your feedback on #272

[nmattia]

Was addressed in #438