app/views/devise/_links.erb
<%- if devise_mapping.omniauthable? %>
<div class="well">
<%# <%- resource_class.omniauth_providers.each do |provider| %>
<%- [:google].each do |provider| %>
<%= link_to "Sign in with #{provider.to_s.titleize}", omniauth_authorize_path(resource_name, provider), class: "btn btn-primary btn-large" %>
<% end -%>
</div>
<% end -%>