chore(travis): allow to build on nightly as well
chore(travis): allow to build on nightly as well

Nightly supports additional features, which are now properly named.
The reason for it to fail recently was the default feature name
`unstable`, which is `nightly` in our case.

dermesser/yup-oauth2,dermesser/yup-oauth2,Narfinger/yup-oauth2