set up a live CI demo on gitlab

[johnnychen94]

There're four changes in this PR, there're all related to CI:

• fix a makefile error: The makefile introduced in #8 should use hard TAB instead of speces
• caching julia packages: adding and buiding julia packages in CI is time-consuming
• remove the installation of pythontex 0.17-dev in CI: I've installed it in the docker image :D

and most importantly,

• add a CI badge to README. (check the README)

Unfortunately, it's not identically this repo which I don't have permission to.

If you want, I can give you the maintainer permission to gitlab demo; or you could fork and set up another demo repo 😄