Removing unused var to quiet warnings.
Skipping XPath perf test on non-JRubies

the original intent was to assert on xpath query performance in the Java
codebase; and running valgrind on this code results in (usually) the
timeout being exceeded on MRI.
Updating list of rubies to test against before release.

Add 2.2.0-preview and drop 1.9.2. Note this doesn't imply (yet) that
we're dropping support for 1.9.2. Yet.
Updating CHANGELOGs with release date.
Version bump to 1.6.4.