Adding bundle exec aliases

Adding alias to map
* rake => bundle exec rake
* rspec => bundle exec rspec
I would propose something that does not conflict with an actual top-level bin name.

What about just aliasing `bundle exec` to `be` that way you can do `be rake` `be rspec`.

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Actually I think what your after is the `rbenv-bundler` gem. You can install it with homebrew `brew install rbenv-bundler` and it will accomplish what your after.