Fix SourceCustom test failure The failure is the following:
1) Vips::SourceCustom can write an image to a user output stream Failure/Error: expect(image.avg).to be_within(0.001).of(109.789) expected 109.41407425505706 to be within 0.001 of 109.789
This has been consistently happening which demonstrate that the value in be_whithin is not too accurate. To avoid this issue let's increase this value and report this issue upstream to see their thoughts.
