Add pyyaml as an optional dependency in travis config
Add pyyaml as an optional dependency in travis config

dhomeier/astropy,saimn/astropy,DougBurke/astropy,stargaser/astropy,lpsinger/astropy,lpsinger/astropy,mhvk/astropy,tbabej/astropy,tbabej/astropy,saimn/astropy,aleksandr-bakanov/astropy,AustereCuriosity/astropy,astropy/astropy,stargaser/astropy,MSeifert04/astropy,MSeifert04/astropy,joergdietrich/astropy,AustereCuriosity/astropy,tbabej/astropy,lpsinger/astropy,kelle/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,larrybradley/astropy,StuartLittlefair/astropy,kelle/astropy,aleksandr-bakanov/astropy,DougBurke/astropy,mhvk/astropy,joergdietrich/astropy,kelle/astropy,StuartLittlefair/astropy,bsipocz/astropy,pllim/astropy,dhomeier/astropy,larrybradley/astropy,tbabej/astropy,dhomeier/astropy,MSeifert04/astropy,dhomeier/astropy,mhvk/astropy,mhvk/astropy,funbaker/astropy,funbaker/astropy,astropy/astropy,stargaser/astropy,kelle/astropy,joergdietrich/astropy,StuartLittlefair/astropy,saimn/astropy,MSeifert04/astropy,tbabej/astropy,bsipocz/astropy,lpsinger/astropy,pllim/astropy,astropy/astropy,DougBurke/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,saimn/astropy,pllim/astropy,dhomeier/astropy,larrybradley/astropy,joergdietrich/astropy,joergdietrich/astropy,mhvk/astropy,saimn/astropy,astropy/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,StuartLittlefair/astropy,funbaker/astropy,kelle/astropy,funbaker/astropy,stargaser/astropy,larrybradley/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,bsipocz/astropy,AustereCuriosity/astropy,DougBurke/astropy,astropy/astropy,AustereCuriosity/astropy