Add truffleruby-head to CI matrix

[stanhu]

This supersedes #70.

[stanhu]

@eregon What do you make of these seg faults with Truffleruby? https://github.com/stanhu/hiredis-rb/runs/7698444091?check_suite_focus=true

[ [ SubstrateSegfaultHandler caught a segfault in thread 0x000055c1fbb29300 ] ]
siginfo: si_signo: 11, si_code: 1, si_addr: 0x0000000000000000

Current timestamp: 1659731297612

General purpose register values:
  RAX 0x00007ff5ab6f3608 points into an aligned TLAB chunk of thread 0x000055c1fbb29300
    is an object of type com.oracle.truffle.llvm.runtime.nodes.memory.store.LLVMI32StoreNodeGen
  RBX 0x00007ff5da6a3ab0 points into the image heap (read-only relocatables)
    is the hub of com.oracle.truffle.llvm.runtime.LLVMLanguage
  RCX 0x00007ff5ab808da8 points into an aligned TLAB chunk of thread 0x000055c1fbb29300
    is an object of type com.oracle.truffle.llvm.runtime.pointer.LLVMPointerImpl
  RDX 0x0000000000000012 is an unknown value
  RBP 0x0000000000000000 
  RSI 0x00007ff5da6a3ab0 points into the image heap (read-only relocatables)
    is the hub of com.oracle.truffle.llvm.runtime.LLVMLanguage
  RDI 0x0000000000000000 
  RSP 0x00007ffdc56b35c0 points into the stack for thread 0x000055c1fbb29300
  R8  0x0000000000000001 is an unknown value
  R9  0x0000000000000012 is an unknown value
  R10 0x0000000000000000 
  R11 0x00007ff5ab808dd8 points into an aligned TLAB chunk of thread 0x000055c1fbb29300
  R12 0x0000000000000001 is an unknown value
  R13 0xffffffffd2f08c88 is an unknown value
  R14 0x00007ff5d8900000 is the heap base
  R15 0x000055c1fbb29300 is a thread
  EFL 0x0000000000010283 is an unknown value
  RIP 0x00007ff5e1ef13d8 points into AOT compiled code 

[eregon]

Could you report an issue to https://github.com/oracle/truffleruby/issues ? That makes it a lot easier to track.
Does the segfault happen every time or only in some runs?

[stanhu]

I'll close this until oracle/truffleruby#2703 is resolved.