zsh: remove vendor/bundle/bin in $path

This is actually not very useful because it means it only works if you run the
bin from the root directory.

Another useful alternatives are:

+ rbenv bundle exec plugin https://github.com/maljub01/rbenv-bundle-exec
+ using smartcd and autostash (part of smartcd) https://github.com/cxreg/smartcd
+ using rbenv-vars https://github.com/sstephenson/rbenv-vars