Use JDK11 for Scala and fix REPL
This differential integrates @livingsilver94's fixes for the Scala REPL, and uses JDK11 for the build and runtime.
In addition, this update uses a different method of specifying the JDK to use, allowing for manual specification of JDK via JAVA_HOME.
Test Plan: Test the interpreter and compile a Hello World program
Reviewers: Triage Team, DataDrake
Reviewed By: Triage Team, DataDrake
Subscribers: DataDrake, livingsilver94
Maniphest Tasks: T6658
Differential Revision: https://dev.getsol.us/D8998