Set RUST_BACKTRACE=1 on Travis CI

Author: messense

.travis.yml

@@ -27,4 +27,5 @@ after_success:
   - travis-cargo coveralls --no-sudo --verify
 env:
   global:
-    secure: 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
+    - secure: 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
+    - RUST_BACKTRACE=1