Fix typo in debugged C source name

I failed to collect any debug info in
https://app.wercker.com/ruby/ruby/runs/mjit-test1/5cda57fcab79a30008f195f6?step=5cda6a4e1090c4000800772e

It seems that it's due to this typo.

wercker.yml

@@ -29,7 +29,7 @@ mjit-test1:
           cat <<'EOS' > /usr/local/bin/mjit-debug-on-fail
           #!/bin/bash
           if ! "$@"; then
-            for f in $(find /tmp -type f -name "ruby_mjit*.c"); do
+            for f in $(find /tmp -type f -name "_ruby_mjit*.c"); do
               echo "[${f}]==="
               cat "$f"
               echo "==="