PROTON-2281 Fix flakiness in ruby-message-spec test (#282)

diff --git a/ruby/spec/message_spec.rb b/ruby/spec/message_spec.rb
index 7976716..2caf5a3 100644
--- a/ruby/spec/message_spec.rb
+++ b/ruby/spec/message_spec.rb
@@ -109,7 +109,7 @@
 
       it "raises an error when the time-to-live is negative" do
         proc {
-          @message.ttl = (0 - rand(1000))
+          @message.ttl = (0 - (rand(1000) + 1))
         }.must_raise(RangeError)
       end