switched coveralls badge to use master branch
switched coveralls badge to use master branch

oittaa/acme-tiny,ushuz/letsencrypt,corokada/acme-tiny,appuio/acme-tiny,appuio/acme-tiny,corokada/acme-tiny,diafygi/acme-tiny,oittaa/acme-tiny,Trim/acme-dns-tiny,rspeed/acme-simple,appuio/acme-tiny