Make require logic rbenv friendly

Rbenv installs its rubies at `~/.rbenv/versions/` by default.

If matching `/versions/` none of the files are required.

Example require paths:

```
/Users/mhfs/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/topsy-1.0.0/lib/topsy/author.rb
/Users/mhfs/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/topsy-1.0.0/lib/topsy/client.rb
/Users/mhfs/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/topsy-1.0.0/lib/topsy/configurable.rb
/Users/mhfs/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/topsy-1.0.0/lib/topsy/default.rb
/Users/mhfs/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/topsy-1.0.0/lib/topsy/errors.rb
```