Use xz tarballs if available

Saves us ~25% bandwidth while downloading the payload. This only applies
to hosts that has the `xz` binary and attempts to use iojs 2.3.2 or newer
(this includes nodejs 4.0+ as well). Older targets are unaffected.

test/fast/teardown

@@ -6,5 +6,5 @@
 
   type setopt >/dev/null 2>&1 && setopt NULL_GLOB
   type shopt >/dev/null 2>&1 && shopt -s nullglob
-  rm -fR v* src alias
+  rm -fR v* src alias test/test-xz
 )