Fix build if secure env vars are unavailable.

.travis.yml

@@ -9,7 +9,9 @@ addons:
     packages:
       - unzip
 before_install:
-  - openssl aes-256-cbc -K $encrypted_ab99677a831c_key -iv $encrypted_ab99677a831c_iv -in deploy_rsa.enc -out deploy_rsa -d
+  - if [ "$TRAVIS_SECURE_ENV_VARS" = true ]; then
+      openssl aes-256-cbc -K $encrypted_ab99677a831c_key -iv $encrypted_ab99677a831c_iv -in deploy_rsa.enc -out deploy_rsa -d;
+    fi
   # The Rust stable toolchain is already installed, ${RUST_NEXT} is added to
   # provide `cargo clippy` and `cargo fmt`.
   - rustup toolchain install ${RUST_NEXT}