fasttracker2: update to b155

Changes since b154:

Beta #155 - 18.05.2019
- The old dithered 4-color FT2 logo in the About screen has been replaced with
  a true color version that I tried to somewhat replicate myself. The old one
  simply didn't look very nice when upscaled...
  Sorry for the lousy changelog for this version!

(fox)