show groups crashes when there are no groups

[bhouse]

[2015-03-09 18:23:06 UTC] DEBUG: Dispatching message to Lita::Handlers::GroupMention#show_groups.
[2015-03-09 18:23:06 UTC] ERROR: Lita::Handlers::GroupMention crashed. The exception was:
Message must have :content
Full backtrace:
/vagrant/vendor/bundle/ruby/2.1.0/gems/flowdock-0.5.0/lib/flowdock.rb:139:in `chat_message'
/vagrant/lita-flowdock/lib/lita/adapters/flowdock/connector.rb:56:in `block in send_messages'
/vagrant/lita-flowdock/lib/lita/adapters/flowdock/connector.rb:55:in `each'
/vagrant/lita-flowdock/lib/lita/adapters/flowdock/connector.rb:55:in `send_messages'
/vagrant/lita-flowdock/lib/lita/adapters/flowdock.rb:38:in `send_messages'