Move Gemfile dependencies inside a test/development group (#1139)
Move Gemfile dependencies inside a test/development group (#1139)

To make it clearer that they aren't used in production.
The gems list has also been sorted alphabetically.

https://bundler.io/guides/groups.html

Closes @W-8619625@.

[skip changelog]
AltSchool/heroku-buildpack-python,Scalingo/python-buildpack,heroku/heroku-buildpack-python,heroku/heroku-buildpack-python,AltSchool/heroku-buildpack-python,Scalingo/python-buildpack