improve startup speed by only calling `compinit -i` once

I don't see the need for calling compinit -i in env-setup just so we can setup compdef for the antigen command, compinit -i is called again in antigen-apply and the user is not likely to be interacting with their shell before the call to antigen-apply has been made.

This change gave me a 58% speedup in startup time (from 1.7s to 0.98s).

Refs #28