Fixes logo URL in README.md

Chrome states:
    "Refused to load the image
    'http://github.com/.../js-by-forio.png?raw=true' because it
    violates the following Content Security Policy directive: ..."

This changes the URL to HTTPS. Protocol-relative URLs don't seem to work
with markdown.
Merge pull request #232 from remram44/fix-readme-logo

Fixes logo URL in README.md (1-char patch)