[ImgBot] Optimize images

*Total -- 5,150.94kb -> 4,481.52kb (13%)

/maths/images/gaussian.png -- 52.26kb -> 38.22kb (26.86%)
/compression/image_data/PSNR-example-comp-10.jpg -- 103.66kb -> 81.56kb (21.32%)
/images/Travis_CI_fail_1.png -- 78.38kb -> 62.91kb (19.73%)
/images/Travis_CI_fail_2.png -- 44.59kb -> 36.97kb (17.08%)
/digital_image_processing/histogram_equalization/output_data/output.jpg -- 115.69kb -> 100.13kb (13.44%)
/arithmetic_analysis/image_data/2D_problems_1.jpg -- 40.42kb -> 35.17kb (12.99%)
/compression/image_data/PSNR-example-base.png -- 4,409.19kb -> 3,839.60kb (12.92%)
/arithmetic_analysis/image_data/2D_problems.jpg -- 57.38kb -> 51.69kb (9.9%)
/digital_image_processing/image_data/lena_small.jpg -- 6.81kb -> 6.17kb (9.32%)
/digital_image_processing/histogram_equalization/image_data/input.jpg -- 58.70kb -> 54.75kb (6.71%)
/digital_image_processing/image_data/lena.jpg -- 101.98kb -> 95.45kb (6.4%)
/compression/image_data/original_image.png -- 81.90kb -> 78.88kb (3.68%)

Signed-off-by: ImgBotApp <[email protected]>

Author: ImgBotApp