Update (optional) asdf requirement in travis config
Update (optional) asdf requirement in travis config

mhvk/astropy,saimn/astropy,saimn/astropy,stargaser/astropy,stargaser/astropy,astropy/astropy,aleksandr-bakanov/astropy,MSeifert04/astropy,larrybradley/astropy,lpsinger/astropy,bsipocz/astropy,saimn/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy,mhvk/astropy,StuartLittlefair/astropy,lpsinger/astropy,bsipocz/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,lpsinger/astropy,MSeifert04/astropy,StuartLittlefair/astropy,bsipocz/astropy,MSeifert04/astropy,saimn/astropy,dhomeier/astropy,larrybradley/astropy,saimn/astropy,dhomeier/astropy,dhomeier/astropy,pllim/astropy,astropy/astropy,stargaser/astropy,larrybradley/astropy,larrybradley/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,pllim/astropy,mhvk/astropy,aleksandr-bakanov/astropy,stargaser/astropy,mhvk/astropy,larrybradley/astropy,MSeifert04/astropy,mhvk/astropy,StuartLittlefair/astropy,pllim/astropy,pllim/astropy,astropy/astropy,dhomeier/astropy,bsipocz/astropy