test: Install older version of bson for JRuby

At this time, bson 5.1.0 is not compatible with JRuby. However, older
versions are compatible. Install the latest version below the
incompatible one.

Author: kaylareopelle

instrumentation/mongo/Appraisals

@@ -6,4 +6,9 @@
 
 appraise 'mongo-2' do
   gem 'mongo', '~> 2.13'
+
+  # TODO: bson 5.1.0 isn't compatible with JRuby at this time
+  if defined?(JRUBY_VERSION)
+    gem 'bson', '< 5.1.0'
+  end
 end