Prepare release v2.0.0

[heroku-linguist]

heroku/java

• Updated heroku/jvm to 2.0.0.
• Updated heroku/maven to 2.0.0.

heroku/java-function

• Updated heroku/jvm-function-invoker to 2.0.0.
• Updated heroku/jvm to 2.0.0.
• Updated heroku/maven to 2.0.0.

heroku/jvm

• This buildpack now requires that an OpenJDK version is specified in system.properties when the buildpack is used standalone (no other buildpack adds jdk to the build plan requires). Users that use this buildpack in a standalone fashion can add a system.properties file to their application with the following contents to restore the old behavior of installing the most recent OpenJDK 8 release: java.runtime.version=8. (#546)

heroku/jvm-function-invoker

• No changes.

heroku/maven

• No changes.

heroku/sbt

• No changes.

heroku/scala

• Updated heroku/jvm to 2.0.0.
• Updated heroku/sbt to 2.0.0.