Bump minimum supported Zsh to v4.3.11
Don't override customized environments in subshells

Only set up environment for the top-level shell, and allow it to be
inherited normally.

A top level shell is usually a login shell, but can also be a
non-interactive, non-login shell in certain cases, such as when
executing an SSH remote command.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Add Rails 4 bin path
Rewrite _rails-command to work from app subdirectories
Do not specify a bundler install path
Git ignore bundler vendor/assets