undefined method `sign_out_via' for Devise:Module

[pcreux]

This commit b93a930 introduced a bug with older versions of devise that does not implement the sign_out_via method.

undefined method `sign_out_via' for Devise:Module (NoMethodError)
/..../active_admin/lib/active_admin/devise.rb:12:in `config'

We should check if devise responds_to sign_out_via before adding the sign_out_via config option.