trello_helper.rb: update card_labels for new gem
trello_helper.rb: update card_labels for new gem

`ruby-trello` gem version 1.5.1 includes a change to the data structure
returned by `card.labels`. This change accounts for that and fixes https://github.com/openshift/sprint_tools/issues/48

danmcp/sprint_tools,dobbymoodge/sprint_tools,dobbymoodge/sprint_tools,openshift/sprint_tools,openshift/sprint_tools,danmcp/sprint_tools