for rails 4
 - device.rb => remove attr_accessible
 - gcm_migrations_generator.rb => remove extend & add next_migration_number
Use "where" method instead of "all" method

Avoid the deplication warning like following;

DEPRECATION WARNING: Relation#all is deprecated. If you want to eager-load a relation, you can call #load (e.g. `Post.where(published: true).load`). If you want to get an array of records from a relation, you can call #to_a (e.g. `Post.where(published: true).to_a`).
Fix the bug of "if" in validates_presence_of :collapse_key
Merge branch 'rails-4'
Merge branch 'remove-deplication-warning-of-relation-all'
Merge branch 'fix-validates-presence-if'