Add Web Apps

Minor change to drone build-image repositories.

Adding Web-apps

Add 3 web applets (#8)

- Add 3 example web applications: Dash (python), R Shiny, and TensorBoard.
- Add a web app section to the tutorials page with these 3 web applets
- Move the link to the source on GitHub prominently at the top of each applet page, and make it open in a new tab

Verifying drone release infrastructure

Update gems (#7)

* Add rvm caveat

* move Jekyll 3.7.x for security reasons

* resolve extract-elems  as git gem

* update docu with todo about extract-elems