Clean up Travis configuration
Clean up Travis configuration

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