Really, really deprecate

[calebhearth]

• Replace README with instructions to use platform-api gem
• Add post_install_message with same information.
• Bump to 0.4.2 (patch version to be included in as many upgrades as possible)

❯ gem install heroku-api-0.4.2.gem

‼️      The heroku-api gem will not work.
‼️      You must instead use the platform-api gem.
‼️      The heroku-api gem communicated with the Legacy API which has been disabled.
‼️      https://devcenter.heroku.com/changelog-items/118
Successfully installed heroku-api-0.4.2
1 gem installed

[geemus]

LGTM.

@dickeyxxx @ransombriggs - any concerns/questions, or shall I merge/release?

[ransombriggs]

@geemus sounds good to me