Legacy ComputerCraft is fun right? Not when the game crashes because you keep Java up-to-date! But, it's a very simple fix, so you will not have to revert to Java 6.

Step 1)
Install the Java 8 Update 11 SDK: http://www.oracle.com/technetwork/java/javase/downloads/java-archive-javase8-2177648.html#jdk-8u11-oth-JPR

Step 2)
Install forge, and make a separate profile for the old Forge.

Step 3)
Go into profile settings, check Executable under Java Settings, and change the path to /Library/Java/JavaVirtualMachines/jdk1.8.0_11.jdk/Contents/Home/bin/java

Step 4)
Play that old ComputerCraft server.