Replace /usr/bin/env with env

Some environments (such as Android) does not have /usr/bin. There are more changes necessary to get oh-my-zsh working in such an environment, but this is a first step and to get feedback!

Is it safe to just replace /usr/bin/env => env?

If not, would it be ok to fallback to just "env" if /usr/bin/env does not exist, or some other approach maybe?