Update release date for 0.9.6
Make lset#inspect produce prettier results.

Remove the redundant "Set:" output from lset#inspect. Along the way, fix a test
failure with MRI 1.8 -- apparently Set#inspect in MRI 1.8 does not produce
sorted output.
Replace usage of Object#id with Object#object_id.

Object#id is deprecated in MRI 1.8 and has been removed in MRI 1.9.
Fix notin test failure with MRI 1.8.

We can't use columns named "id" in MRI 1.8.