Support for heroku pipelines

[venkatd]

In order to be able to use Heroku CI properly, I had to do the following:

1. Make use of the #131 fork because erlang was not properly getting installed
2. Set MIX_ENV properly in my app.json like below

  "environments": {
    "test": {
      "env": {
        "MIX_ENV": "test"
      }
    }
  }

I'm happy to issue a PR to make #2 unnecessary if you can point me to where to make the change. The error I got was not obvious so it took me a while to realize MIX_ENV was set to prod instead of test.

@HashNuke

[HashNuke]

@venkatd Please let me know if this is something you can still contribute a change/fix for. I have not used Heroku Pipelines much.