Replace Category.find(:all) with Category.all because find(:all) is deprecated

This commit removes the warning as follows,
DEPRECATION WARNING: Calling #find(:all) is deprecated. Please call #all directly instead.
Merge branch 'replace_deprecated'

Merge branch 'replace_deprecated' of https://github.com/kaznum/FlickrFavoriteManager into kaznum-replace_deprecated

Conflicts:
	app/views/favorite_images/_form.html.erb