Add less restrictive gem spec for mongo gem

It looks like latest features (including jruby 10 enablement) will require the
latest minor release of the mongo gem.

Specifically:

The latest mongo gem relaxes the range for the ruby-bson gem. This would allow
the uptake of mongodb/bson-ruby#353 if it is accepted.

Author: donoghuc

instrumentation/mongo/Appraisals

@@ -5,5 +5,5 @@
 # SPDX-License-Identifier: Apache-2.0
 
 appraise 'mongo-2.13' do
-  gem 'mongo', '~> 2.13.0'
+  gem 'mongo', '~> 2.13'
 end