Use RUBYLIB instead of RUBYOPT to add the bundle lib path to ruby $LOAD_PATH.

This allows `bundle exec` to be run when the bundler gem is at a path that includes spaces (which is pretty common in windows environments).
Remove unnecessary quotes around bundler lib path. 

As suggested by gnufied.
Improve RUBYLIB example in exec spec.
Merge pull request #2536 from equinux/master

Use RUBYLIB instead of RUBYOPT to add bundle lib path to ruby $LOAD_PATH.