Improve how ruby features are required

* Don't require bcrypt explicitly (done by bundler);
* Regroup require calls in application configuration.

app/models/sound.rb

@@ -1,5 +1,3 @@
-require 'fileutils'
-
 class Sound < ActiveRecord::Base
   belongs_to :track
 

app/models/user.rb

@@ -1,5 +1,3 @@
-require 'bcrypt'
-
 class User < ActiveRecord::Base
   include BCrypt