Updates pluck:update_pics rake task

When attempting to update profile pictures that have gone stale, there are some cases where a resource will not be found (e.g., the tweet or the user account was deleted). Previously, this caused the task to fail on the first missing resource preventing all subsequent profile pictures from being updated. Moving the API call into a `begin` block and `rescue`ing by stepping to the `next` iteration fixes this.

I'll have to figure out what to do with the few user profile pictures I can't update. I can either add a stock photo and see if I can a photo of the user using some other means.