using the stable spin's once implementation

[brunoczim]

I am opening a different pull request since this is different from #122
We should now be able to have no_std lazy_static with stable Rust.

[KodrAus]

Thanks @brunoczim!

Would you also like to update our .travis.yml so that we check this feature on the latest stable too:

matrix:
  include:
    - rust: stable
      script:
        - cargo test
        - cargo test --features spin_no_std

[brunoczim]

Done.

Please tell me if I did something wrong with travis. I am more used to gitlab CI :P

[KodrAus]

This looks good to me! Thanks again.