Heroku changes

[darbyfrey]

• The Heroku Logplex doesn't support logging to JSON format, this change defaults Napa to use the :basic format when on Heroku.
• When a slug is built for Heroku it strips out all the git information, so we don't have access to git to find the current sha. This change remove the call to git and returns the value from an ENV which can be set during a post commit hook if desired. Alternately, since the sha is easily accessible in other ways in Heroku it may not be necessary to include in the health check payload.

@bellycard/platform

[umtrey]

+1

[shaqq]

+1

it might be worth creating a napa-heroku gem at some TBD point, since there may be a set of convenience methods that makes using napa + heroku = amazing.

anywho, this PR looks to go. thanks @darbyfrey!

[heymackey]

+1