Edit Ocaml module

Use $OPAMROOT in modules/ocaml/init.zsh

modules/ocaml/init.zsh

@@ -5,10 +5,15 @@
 #   Sebastian Wiesner <lunaryorn@gmail.com>
 #
 
+# Set $OPAMROOT
+if [[ -z "$OPAMROOT" ]]; then
+    export OPAMROOT="${HOME}/.opam"
+fi
+
 # Return if requirements are not found.
-if [[ ! -f "$HOME/.opam/opam-init/init.zsh" ]]; then
+if [[ ! -f "$OPAMROOT/opam-init/init.zsh" ]]; then
   return 1
 fi
 
 # Initialize OPAM.
-source "$HOME/.opam/opam-init/init.zsh"
+source "$OPAMROOT/opam-init/init.zsh"