Replaced the 'unicode' gem with 'unf' so it'll work with JRuby.

Fixes #2279: Remove dependency on "unicode" gem.
Made the 'unf' dependency optional.

If you don't have 'unf' installed and you use an AWS adapter, you'll get a message about how your strings might not be properly escaped.  If the gem is available, we'll use it.
Merge pull request #2320 from nirvdrum/replace_unicode_with_unf

Replaced the 'unicode' gem with 'unf' so it'll work with JRuby.