Remove non-directories from rollback slot list
Remove non-directories from rollback slot list

Addressing #4586. The bug stops Spacemacs from listing rollback slots if
there exists a file (i.e. non-directory) in the rollback directory.
e.g. .emacs.d/.cache/.rollback/.DS_Store (common in OS X)

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs