.zshrc: Use simpler pyenv initialization

Instead of using it from a local git checkout I'll just use it from
Homebrew.

.zshrc

@@ -79,19 +79,8 @@ if [[ -d ~/.nvm ]]; then
     . ~/.nvm/nvm.sh
 fi
 
-# Enable pyenv
-# See: https://github.com/yyuu/pyenv#basic-github-checkout
-if [[ -d ~/.pyenv ]]; then
-    export PYENV_ROOT="$HOME/.pyenv"
-    PATH="$PYENV_ROOT/bin:$PATH"
-
-    eval "$(pyenv init -)"
-    # optionally enable pyenv-virtualenv
-    # See: https://github.com/yyuu/pyenv-virtualenv
-    if [[ -d ~/.pyenv/plugins/pyenv-virtualenv ]]; then
-        eval "$(pyenv virtualenv-init -)"
-    fi
-fi
+# Initialize pyenv if it exists
+if which pyenv > /dev/null; then eval "$(pyenv init -)"; fi
 
 # If a private bin directory exists, add it to PATH
 [[ -d ~/bin ]] && PATH=$PATH:~/bin