Fix uses of Array#- on Rubinius.

Apparently it doesn't guarantee the order of the output array.
Fix ArgumentError handling on Rubinius.
Merge branch 'stable' of github.com:nex3/sass into stable
Mention previous commit in the changelog.
Slightly better ArgumentError detection for JRuby.
Support JRuby 1.7.

Closes #600.
Fix error-character quoting for encoding errors on Rubinius 2 in 1.9 mode.
Have MultibyteStringScanner delegate to StringScanner on Rubinius.

Closes #623
Add a changelog entry about Rubinius stuff.
Merge remote-tracking branch 'origin/stable' into stable