Alias all ruby bins in your rvm install to b<bin>, which bundle exec's it.

bundle_exec_aliases.zsh

for i in $(rvm gemdir)/bin/*; do
    bin=$(basename $i)
    if (! which $bin &>/dev/null); then
        echo "Aliasing b$bin to bundle exec $bin"
        alias b$bin="bundle exec $bin"
    fi
done