Revert adding /usr/local/bin to path in zshenv

* Adding `/usr/local/bin` to path in `~/.zshenv` causes it to be in my path twice one right after the other. `/usr/local/bin` is already added to the path in the correct order on OSX Yosemite by the `/etc/paths` file. 

* Remove /usr/local/bin from path export in zshenv