TST: Add base branch check.
TST: Add base branch check.

Use default branch as E. M. Bray suggested.

[skip circleci]

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