Use Schematic-generated heroku-go for API access

Replaces Blake's outdated heroku-go with the Schematic-generated one
which seems to have consensus as the way forward for Heroku API access
in Go.

Strips out logic for reading from `~/.netrc` and instead prefers the
more explicit system of having a `HEROKU_ACCESS_TOKEN` passed in on
start-up.
Merge pull request #1 from fdr/brandur-schematic

Use Schematic-generated heroku-go for API access