Pin ABCL to 1.7.1 on JDK 8 for testing
ABCL 1.8.0 has an issue where the PID of a process cannot be retrieved, causing a failure in test-run-program-unix.script. This has been reported to and reproduced by ABCL devs. Additionally, ABCL 1.8.0 fails test-around-compile.script. I'm still trying to figure out if this is ABCL's fault or ASDF's fault, but it's likely ABCL's given that it works in 1.7.1. Pin to JDK 8 because newer JDKs have issues even launching external processes (that issue has been fixed in 1.8.0).