fix broken gm dependency

[shime]

since this module doesn't support gm >= 1.0.0, let's specify it in package.json.

[shime]

also, this is what I'm getting when running the readme example

[ [ 1, [ 73, 46, 39 ] ],
  [ 1, [ 37, 19, 13 ] ],
  [ 1, [ 42, 30, 23 ] ],
  [ 1, [ 45, 34, 30 ] ],
  [ 1, [ 55, 39, 29 ] ],
  [ 1, [ 57, 34, 18 ] ],
  [ 1, [ 57, 38, 25 ] ],
  [ 1, [ 62, 41, 27 ] ],
  [ 1, [ 64, 46, 36 ] ],
  [ 1, [ 44, 24, 14 ] ],
  [ 1, [ 65, 48, 41 ] ],
  [ 1, [ 71, 55, 43 ] ],
  [ 1, [ 76, 56, 51 ] ],
  [ 1, [ 79, 62, 58 ] ],
  [ 1, [ 92, 70, 59 ] ],
  [ 1, [ 105, 83, 70 ] ],
  [ 1, [ 121, 99, 89 ] ],
  [ 1, [ 136, 115, 103 ] ] ]

although frequencies shouldn't be all 1, since I've passed true.