fix: js/Promise.js does not exists anymore, use es6-promise instead

This commit fix an error introduced by #57 when using node (or browserify):

  - The file `js/Promise.js` no longer exists but is sometimes required by `jsonld.promisify()` 
  - `js/Promise.js` can be replaced by `es6-promise`
Merge pull request #59 from nopnop/update_promise_dependency

fix: js/Promise.js does not exists anymore, use es6-promise instead