Add libm to no_std CI

.travis.yml

@@ -22,6 +22,7 @@ matrix:
         - rustup target add $TARGET
       script:
         - cargo build --verbose --target $TARGET --no-default-features --features i128
+        - cargo build --verbose --target $TARGET --no-default-features --features libm
     - name: "rustfmt"
       rust: 1.31.0
       before_script: