Change `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.
Merge pull request #2212 from bsiegel/bundler_bi_function

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