Update chunky_png dependency to version 1.2.0
Update chunky_png dependency to version 1.2.0

openeducation/travis-ci,fxposter/travis-ci,fxposter/travis-ci,flores/travis-ci,fxposter/travis-ci,openeducation/travis-ci,flores/travis-ci,flores/travis-ci,openeducation/travis-ci