add no_std build test to travis

Author: apoelstra

.travis.yml

@@ -14,5 +14,6 @@ script:
   - cargo test --verbose
   - cargo test --verbose --features "serde"
   - cargo build --verbose --features "fuzztarget"
+  - cargo build --verbose --no-default-features
   - if [ "$(rustup show | grep default | grep beta)" != "" ]; then cargo install --force cargo-web && cargo web build --target=asmjs-unknown-emscripten && cargo web test --target=asmjs-unknown-emscripten --nodejs; fi
   - if [ "$(rustup show | grep default | grep stable)" != "" ]; then cd fuzz && cargo test --verbose && ./travis-fuzz.sh; fi