Fixes a regression in update process added in #115

As #115 migrated list functions, after updating Oh My Fish code
the omf.list_installed_packages gets removed, but is still being
used by the update code.

This commit fixes it by providing a wrapper function.

pkg/omf/cli/omf.list_installed_packages.fish

+# Backwards-compatible wrapper function
+# TODO: Remove it after 2015, December 13
+function omf.list_installed_packages
+  omf.packages.list --installed --plugin
+end