Add rake explicitly to gemfile

[patcon]

Seems that heroku scheduler always appends commands with bundle exec, and for some reason this fails on heroku without explicitly added rake to gemfile.

related SO thread: https://stackoverflow.com/questions/8176282/bundle-exec-rake-dbmigrate-causes-cant-find-executable-rake-error
my fail logs: https://gist.github.com/patcon/8c4154b294e03b0902f60efbcda20ac0

Steps to reproduce:

1. open console in heroku: heroku run bash
2. Run: bundle exec rake prog:channels:sync

We should add this.

[patcon]

Closing in favour of open PR with commit