Allow nightly builds to fail.

Clippy is currenty broken, so our nightly build can't succeed.

.travis.yml

@@ -4,6 +4,9 @@ rust:
     - stable
     - beta
     - nightly
+matrix:
+  allow_failures:
+    - rust: nightly
 before_script:
     - pip install 'travis-cargo>=0.1.0,<0.2.0' --user
     - pip install 'CppHeaderParser>=2.7.2,<3.0.0' --user