RUBY-2351 Increase minimum Ruby version to 2.4 (#237)

Author: comandeo-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 library is compatible with MRI >= 2.3 and JRuby >= 9.2.
+The BSON library is compatible with MRI >= 2.4 and JRuby >= 9.2.
 
 Use With ActiveSupport
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