Timeseries Widget: convert tick Enum to Arr for .last

Avoid NoMethodError on tick.last by converting it to array

    NoMethodError - undefined method `last' for #<Enumerator: 1377191030..1377191330:step(10)>
	/var/lib/gems/1.9.1/gems/fnordmetric-1.2.9/lib/fnordmetric/widgets/timeseries_widget.rb:35:in `data'
	/var/lib/gems/1.9.1/gems/fnordmetric-1.2.9/lib/fnordmetric/widget.rb:75:in `render'