Use conventional zsh dotfile names

* move rebenv initialization from .zlogin to .zshrc
* rbenv documentation recommends .zshrc
* less files for us to manage in our home directory

* use 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)