Fixed strong parameters for latest Rails 4 releases
depending on bcrypt gem (not bcrypt-ruby)

Conflicts:
	gemfiles/Gemfile.rails-4.1-stable
Travis configuration update

- use system libraries to build Nokogiri
- test with Ruby 1.9.3 and 2.1.x
- don't test Rails-head with Ruby 1.9.3
Updated README [ci skip]
Validate host before redirect on login

Janus used to blindly redirect to `params[:return_to]` whenever the
host was the request.host (hence no security issues), but the
developer was missing the possibility to interupt the redirect if
the host isn't accessible for some reason.
Fix Travis build matrix
v0.9.1