use correct link for Travis CI badge
use correct link for Travis CI badge

muzykabart/spree_braintree_vzero,muzykabart/spree_braintree_vzero,spree-contrib/spree_braintree_vzero,spree-contrib/spree_braintree_vzero,spree-contrib/spree_braintree_vzero,muzykabart/spree_braintree_vzero