fix CSRF vulnerability. prepare 1.5.0 release
trigger travis CI build
config travis to build all branches
remove now redundant per-request state from signed_request flow (it will fail CSRF)
this does not provide authentication, should not be used
remove references to state param in docs
bump 1.5.1
Merge branch '1-5-stable'

Conflicts:
	README.md
	lib/omniauth/facebook/version.rb
	lib/omniauth/strategies/facebook.rb
	test/test.rb