Switch to using ddollar/heroku-buildpack-multi

[andylolz]

https://github.com/andylolz/heroku-buildpack-python-extras appears to be broken now. Rather than fix it, it’s probably better to switch to use https://github.com/ddollar/heroku-buildpack-multi

Here are the steps to doing this:

• run:

  heroku buildpacks:set https://github.com/ddollar/heroku-buildpack-multi.git

  …to use this new buildpack

• merge this pull request (or pull it locally to test it, I guess)
• push to heroku

NB The python buildpack happens to run bin/post_compile, which is really handy because lesson_format utilises this. So as long as the python buildpack is last in the .buildpacks file, this works (since bin/post_compile is dependent on stuff in the other two buildpacks). I am noting this here because it seems like something that could break in the future.

Fixes (well, hopefully) #178.

[martinpeck]

I was going to try taking the buildback from which you based yours (https://github.com/localmed/heroku-buildpack-python-extras) and seeing if that worked. Do you think that's going to be broken in similar ways?

[andylolz]

Do you think that's going to be broken in similar ways?

Similar ways, plus more ways as well.

• Similar ways because it hasn’t been updated for ages (not since I forked it, anyway) so it almost certainly has all the same problems my fork has
• More ways because it’s missing anything ruby-related (that was why I forked it) < This isn’t true! I have no idea why I forked it, since all I appear to have done is remove stuff. Just to speed up the build I suppose.

But anyway, this ddollar/heroku-buildpack-multi is a much more robust solution. And ddollar is was a heroku person anyway (creator of foreman by the looks of things).

[martinpeck]

So, this worked fine on a new heroku app, but failed on the existing one.

[martinpeck]

wait...hold that thought I'm an idiot...

[martinpeck]

Idiot status confirmed. Deployment worked fine. Thanks @andylolz!

[andylolz]

Great! :)