Update deprecated find_or_create_by_ method

This PR fixes Issue #53.

```
Started POST "/recommendations/plan_to_watch" for 127.0.0.1 at 2014-06-14 02:28:01 -0700
Processing by RecommendationsController#plan_to_watch as */*
  Parameters: {"anime"=>"mirai-nikki-tv"}
  [1m[36mUser Load (1.7ms)[0m  [1mSELECT  "users".* FROM "users"  WHERE "users"."id" = 1  ORDER BY "users"."id" ASC LIMIT 1[0m
  [1m[35mAnime Load (1.5ms)[0m  SELECT  "anime".* FROM "anime"  WHERE "anime"."slug" = 'mirai-nikki-tv'  ORDER BY "anime"."id" ASC LIMIT 1
Completed 500 Internal Server Error in 12ms

NoMethodError - undefined method `find_or_create_by_anime_id_and_user_id' for #<Class:0x007f807084b6e0>:
```

The deprecated method `find_or_create_by_...` is still being used in certain places but they seem to be unused, below are the following places:

* `admin_controller#invite_to_beta`
* `watchlist_controller.rb`
* `import.rake#casting`

@vikhyat, do you want me to patch these as well? or are you planning on ripping them out.