Remove `attr_accessible` for Rails 4 compatibility.

Using this with Rails 4 raises an error on `attr_accessible` as it is
removed, in favour of `strong_params`.

The onus is now on the implementer to only permit `name` and
`redirect_uri` when creating an authorization.
Merge remote-tracking branch 'felixclack/rails-4-compatibility'

* felixclack/rails-4-compatibility:
  Remove `attr_accessible` for Rails 4 compatibility.