reverting ' ' around nii.gz because it wasn't working (#661)

Author: stebo85

neurodocker/templates/freesurfer.yaml

@@ -71,7 +71,7 @@ binaries:
  # default freesurfer options
  FS_OVERRIDE: '0'
  FIX_VERTEX_AREA: ''
- FSF_OUTPUT_FORMAT: 'nii.gz'
+ FSF_OUTPUT_FORMAT: nii.gz
  # mni environment requirements
  MINC_BIN_DIR: '{{ self.install_path }}/mni/bin'
  MINC_LIB_DIR: '{{ self.install_path }}/mni/lib'