Update helm-projectile-sources-list priority

Prioritize helm-source-projectile-buffers-list and
helm-source-projectile-recentf-list higher, since those two have smaller
candidate space. Otherwise, those two will never be used because
helm-source-projectile-files-list is a superset of both, and candidates
in helm-source-projectile-files-list are always displayed first, making
the two smaller sources almost never be used, unless explicitly move to
that source by arrow keys which is less convenient.