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.

.travis.yml

@@ -2,6 +2,7 @@ language: rust
 sudo: required
 rust:
 - stable
+- nightly
 before_script:
 - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
 script:
@@ -13,4 +14,5 @@ after_success:
 - travis-cargo coveralls
 env:
   global:
+    TRAVIS_CARGO_NIGHTLY_FEATURE: nightly
     secure: Es8/QExA+XfWaTi2wQqpJL+yc4d3yU9/dfCpVFhyWpu+2XXIvbPbdqFuiZG6QVGaYOmwjlFJ5gCsAqxliG2wzVHkv+KhA1yeVn4ia41Vmx1mJlEU+O7ZNldfgjWemWKTD5kTEFtTBItswKUFH9LsFQ5sztV6/7I9kZZJAxWiXjs=