travis: remove python2.6-tk
travis: remove python2.6-tk

bsipocz/pyspeckit,mikelum/pyspeckit,e-koch/pyspeckit,allisony/pyspeckit,pyspeckit/pyspeckit,low-sky/pyspeckit,allisony/pyspeckit,vlas-sokolov/pyspeckit,jpinedaf/pyspeckit,jpinedaf/pyspeckit,mikelum/pyspeckit,dinossimpson/pyspeckit,e-koch/pyspeckit,keflavich/pyspeckit,low-sky/pyspeckit,dinossimpson/pyspeckit,vlas-sokolov/pyspeckit,keflavich/pyspeckit,keflavich/pyspeckit-obsolete,pyspeckit/pyspeckit,bsipocz/pyspeckit