Check if the file is ignored before adding to cache

* projectile.el (projectile-cache-current-file): Add
  to `projectile-ignored-file-p' as an additional predicate
  against adding the file to the cache.

This fixes a problem of "file.~HEAD~2~" (result of
`vc-revision-other-window') being added to the cache although
".projectile" has:

    -/*~