Don't build no_std on stable

.travis.yml

@@ -15,7 +15,12 @@ install:
   - rustup component add rust-src
   - rustup target add thumbv7m-none-eabi
 
+# TODO: Remove check as soon as Rust 2018 Edition is stable
 script:
   - cargo build
   - cargo test
-  - cd no-std-example && cargo build
+  - |
+    if [[ "$TRAVIS_RUST_VERSION" != "stable" ]]; then
+    cd no-std-example
+    cargo build
+    fi