Update to v5.0.0-beta3

* `rake 'update[v5.0.0-beta3]'`
* Update version in README.md and version.rb

Author: glebm

README.md

@@ -20,7 +20,7 @@ Please see the appropriate guide for your environment of choice:
 Add `bootstrap` to your Gemfile:
 
 ```ruby
-gem 'bootstrap', '~> 5.0.0.beta2'
+gem 'bootstrap', '~> 5.0.0.beta3'
 ```
 
 Ensure that `sprockets-rails` is at least v2.3.2.

assets/javascripts/bootstrap-sprockets.js

@@ -10,6 +10,7 @@
 //= require ./bootstrap/collapse
 //= require ./bootstrap/dropdown
 //= require ./bootstrap/modal
+//= require ./bootstrap/offcanvas
 //= require ./bootstrap/tooltip
 //= require ./bootstrap/popover
 //= require ./bootstrap/scrollspy