Support new elm 0.19

[izderadicka]

By default now elm 0.19 is installed and cannot compile this project easily (as project json structure has changed). I update Makefile, which is relatively easy it was not enough.

[hatzel]

Sadly this is not an option without significant refactoring, as elm-mdl is no longer supported in 0.19, see: debois/elm-mdl#377

[izderadicka]

OK, understood. I uninstalled 0.19 and installed 0.18 and now it's working fine.
Then I would recommend to update readme - so people know instantly to install older elm version.
Maybe one small thing - in Makefile run use python3 command (python 2 does not have http.server module).