fix library loading for non-rbenv users.

This should fix soywiki `0.9.8.1` for **rvm** (or more general non-rbenv users).

(and it should finally fix #38)

@RobrechtDR i've released a pre version of `0.9.8.2` containing this patch, could you please try this and
see if it works for you?

You can install it with `gem install soywiki --pre`