Change bundler plugin's `bi` alias to a function

Only check the bundler version when we call bi. This fixes two issues:
First, if there is no bundler available system-wide this will cause an
error each time zsh loads. Second, if new bundler is installed
system-wide but you change into an rbenv with an older version, the
alias will no longer work.

This fixes the bug / incompatibility introduced in https://github.com/robbyrussell/oh-my-zsh/pull/2107