RUBY-1961 Require Ruby 2.3+ in bson-ruby (#129)

* RUBY-1961 Require Ruby 2.3+ in bson-ruby

* Remove Ruby 1.9 compatibility added in #120

Author: p-mongo

docs/tutorials/bson-v4.txt

@@ -34,7 +34,7 @@ To install the gem with bundler, include the following in your Gemfile:
 
     gem 'bson', '~> 4.0'
 
-The BSON gem is compatible with MRI >= 1.9.3, JRuby >= 9.1, and Rubinius 2.5.x.
+The BSON gem is compatible with MRI >= 2.3 and JRuby >= 9.1.
 
 Use With ActiveSupport
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