Revert "zsh: remove rbenv init from .zshrc"

Unfortunately, rbenv doesn't work as consistently as I had thought when
I original crafted this commit. I'm going to revert it for now to get
some predictability back into the few Ruby things I still have to do.

This reverts commit e7ad860.

.zshrc

@@ -19,6 +19,11 @@ if [[ $- == *i* ]]; then
   fi
 fi
 
+# Load rbenv if available
+if which rbenv > /dev/null; then
+  eval "$(rbenv init -)"
+fi
+
 export EDITOR=vim
 export VISUAL=vim