Remove nightly-only checks from travis-ci

These are no longer necessary as pyo3 builds on stable now.

Author: vbarrielle

.travis.yml

@@ -19,7 +19,6 @@ script:
     - cargo test --release --all --verbose
     - cargo run --example heat
     - ./.travis_rustfmt
-    - ./.travis_nightly
 
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   email: