Heroku complains about missing gem mini_portile2

[amhlaobh]

When deploying to Heroku, it complains about "mini_portile2" missing.

remote:        section: 'mini_portile2'
remote:  !
remote:  !     Failed to install gems via Bundler.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.

A solution is to add this line to Gemfile, e.g. in the group :production do section:
gem 'mini_portile2', '~> 2.1'

And maybe install this gem locally:
gem install mini_portile2