[ci skip] tweak readme
Issue #2: pick up changes from SIXX.310-PaulDeBruicker.182 to get SIXX working with XMLSupport version 1.1.8 for GemStone2.x

Mainly involved changing #isNil sends to #isEmptyOrNil.

Used #isEmptyOrNil rather than #isEmpty because someone may want to use the older version of XMLSupport, which returns #nil for missing attibutes/elements rather than the empty String.
Issue #2: add 2.4 back to lineup
Merge branch 'dev' of github.com:glassdb/SIXX into dev
Merge branch 'dev'