update tests

Usually you want to be less specific about what the problem is. So you can go for something like "Invalid email or password." I'm sure there are other examples you can use as well. Also when rendering you should use `flash.now[:error]` to prevent the flash message from being passed on the the next request. Further for bootstrap 3 you have to use new classes so `flash[:error]` becomes `flash[:danger]`. See [here](http://getbootstrap.com/components/#alerts). Finally, you don't have to use `elsif` and `else` you can just do:
```ruby
else
  render :new
  flash[:danger] = "Invalid email or password."
end
```