Remove README note about Heroku access

[Suspenders]' (the generator used to create the Trailmix Rails app)
`./bin/setup` script doesn't "fail" in a non-zero exit status sense.

[Suspenders]: https://github.com/thoughtbot/suspenders

The `heroku join` lines are protected by `if` statements:

```bash
if heroku join --app trailmix-staging &> /dev/null; then

if heroku join --app trailmix-production &> /dev/null; then
```

The failing branch of the conditionals used to print messages to `stdout`:

> Ask for access to the "trailmix-staging" Heroku app
> Ask for access to the "trailmix-production" Heroku app

This commit removes those messages to avoid a situation where
open-source contributors ask for access to the Heroku apps.