Add rubocop to the list of bundled_commands

Tailor is already in the list for ruby style and syntax checking, but
rubocop has far surpassed it in popularity and is a better default.
On github, rubocop currently has 10x-20x the contributors, forks, and
stars.

plugins/bundler/bundler.plugin.zsh

@@ -25,6 +25,7 @@ bundled_commands=(
   rainbows
   rake
   rspec
+  rubocop
   shotgun
   sidekiq
   spec