just warn on missing FFI support when jruby fails to load FFI 

fixes #50

since krypt does work with having native support for JRuby disabled, it also shoud work in case FFI can not be loaded due to various possible reasons. see https://github.com/jruby/jruby/issues/1800

this is just a suggestion on how to fix it. but it would be nice to see a fix for this on the next jruby-1.7.14 release ;)