Move rebenv initialization from .zlogin to .zshrc

* rbenv documentation recommends .zshrc
* less files for us to manage in our home directory
User zshrc instead of zprofile

* zprofile is just an alternative for ksh fans (source)[http://zsh.sourceforge.net/Intro/intro_3.html]
* Tim Pope recommends moving /etc/zshenv to /etc/zshrc(source)[https://github.com/tpope/vim-rbenv]