Update Janus::TestHelper

Makes the `janus` instance available, so we can interact with it in
tests. Renames the assert_not_authenticated method as
refute_authenticated which fits better with minitest (assert_not is
still available as an alias).
♡ minitest

Use refute instead of assert_not
update .gitignore
valid_password? raises when password is empty #19
Better Change Log

[ci skip]
Rails 4.2 compatibility
depending on bcrypt gem (not bcrypt-ruby)