use TLS for external resources

[InPermutation]

3 changes, in decreasing order of importance:

• Heroku automatically sends you to the HTTPS version of the app when you install with Heroku Button.
  Then, Firefox (since version 23) blocks the download of Reenie Beanie, and apparently replaces it with Comic Sans MS?!
• The changes in README.md are mostly irrelevant because of https://help.github.com/articles/why-do-my-images-have-strange-urls/, but it's good practice, in case it ends up getting rendered somewhere that isn't GitHub.
• I'm not sure if the Placekitten one is even viewable from a browser -- I didn't bother checking. The HTTPS version of the resource does load, however, so it probably won't break anything.

[Koronen]

Thanks @InPermutation!