There is no JRuby version 1.9; this change should avoid confusion

Please be more clear when specifying which version of JRuby mongo_mapper is tested against.

I could figure out what you meant, but only after an `rvm get latest` and quick head scratch while wondering how you guys test against 1.9 when JRuby is only on 1.7.
There is no version 1.9; this change should avoid confusion.