Fix the asset path for the favicon in _head.

The favicon_link_tag method now adds the /asset/ path automatically.
This removes the need for the image_path method.
Fix the spree_auth_devise overrides not being loaded.

When spree_auth_devise_bootstrap was split into a separate internal gem
to this repo I forgot to 'require' it in the parent gem.

spree_auth_devise_bootstrap dependencies were screwy too.
The registration step should match the functionality of spree_auth_devise.

The registration step was previously changed to show the new
registration form rather than the existing user login form. However,
spree_auth_devise shows them the other way around.

I'm changing the functionality back to facilitate the merging of
spree_bootstrap_frontend into spree and spree_auth_devise. Having the
same functionality, just with a different look, will make the merge
less troublesome for users.
Fix the circular dependency between spree_auth_devise_bootstrap and spree_bootstrap_frontend.

spree_auth_devise_bootstrap should point at spree_frontend_bootstrap.
Remove spree_frontend_bootstrap's dependency on spree_auth_devise_bootstrap.

spree_frontend_bootstrap should be useable without spree_auth_devise_bootstrap.
Lock the Spree development dependencies to the last merged commit.

This avoids the situation where something has changed in one of the
upstream dependencies that hasn't yet been merged into this repo that
causes a breakage.

This won't lock the dependencies when spree_bootstrap_frontend is
used as a gem. It's only useful for development of this gem.