update fmriprep version to 1.5.6

[jrasero]

Hi,

this gear currently uses fmriprep 1.5.5.

However, this version is problematic if you try to apply susceptibility distortion correction to the EPI images without fieldmaps using the --use-syn-sdc and --ignore fieldmaps options, due to an existing bug. Please, have a look at:

nipreps/fmriprep#1941

Fixing this issue is important for those who, as I said, need to apply distortion correction to the EPI images in their projects but the fieldmaps are not available or have some deficiencies.

fmriprep 1.5.6 appears to have fixed this issue https://github.com/nipreps/fmriprep/releases/tag/1.5.6