RUBY-1961 remove JRuby 9.1 support in bson-ruby due to it becoming an undue burden (#145)

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 >= 2.3 and JRuby >= 9.1.
+The BSON gem is compatible with MRI >= 2.3 and JRuby >= 9.2.
 
 Use With ActiveSupport
 ----------------------