fix(require): make default require work with bundler

This is a partial revert of commit 7b1cd61.

We're accepting both:

- the recommended `require 'tanker/identity'` (see: https://guides.rubygems.org/name-your-gem/)
- the default `require 'tanker-identity'` that bundler uses

Author: maximerety

README.md

@@ -73,7 +73,7 @@ A secret identity.
 The server-side pseudo-code below demonstrates a typical flow to safely deliver identities to your users:
 
 ```ruby
-require 'tanker/identity'
+require 'tanker-identity'
 
 # 1. store these configurations in a safe place
 app_id = '<app-id>'

lib/tanker-identity.rb

@@ -1,3 +1 @@
 require 'tanker/identity'
-
-warn "DEPRECATION WARNING: Using `require 'tanker-identity'` is deprecated in favor of `require 'tanker/identity'`"