message: Bump png version
  This fixes 16bpc tests.
diff: @@ -26,7 +26,7 @@ num-rational = { version = "0.2.1", default-features = false }
  num-traits = "0.2.0"
  gif = { version = "0.10.0", optional = true }
  jpeg = { package = "jpeg-decoder", version = "0.1", default-features = false, optional = true }
  -png = { version = "0.15", optional = true }
  +png = { version = "0.15.2", optional = true }
  scoped_threadpool = { version = "0.1", optional = true }
  tiff = { version = "0.4.0", optional = true }
  zerocopy = "0.2.8"
  