Update submodule oh-my-zsh 15e6906...3b797e1

* oh-my-zsh 15e6906...3b797e1 (5):
  > github plugin: use `whence -p git` for vcs_info
  > vcs_info: only enable git, dynamically set check_for_changes
  > theme: use `command git` instead of `git`
  > prompt: remove unnecessary color toggle
  > Honor ':vcs_info:*:prompt:*' 'check-for-changes'