Fix $LOAD_FEATURES paths for 1.8

After 613673a, the paths in `$LOAD_FEATURES` are formatted incorrectly in 1.8 mode.  This causes Travis failures in all `RUBYSPEC_LANG_VER='1.8'` suites. [See here for example](https://travis-ci.org/jruby/jruby/builds/30531697).

Travis after this fix looks like [this](https://travis-ci.org/dmarcotte/jruby/builds/30515058) (not green yet... but greener.  The remaining reds are pre-existing and unrelated to this fix.)

@ratnikov please chime in if I've missed something here!