The rate of those lacking health insurance is plummeting in Maine and nationally, according to a Gallup poll released Tuesday. Maine’s uninsured rate declined from 16.1 percent in 2013 to 11.6 percent in 2014, according to the national polling firm. Nationally, those without insurance dropped from 17.3 to 13.8 percent of the population. The survey represents the first time the polling firm captured results for the first full year that the Affordable Care Act’s marketplace was in effect, interviewing people from January through Dec.