[meta] Fix ZSH configuration on OS X

Add `/usr/local/bin/zsh` to `/etc/shells` and fix the shell in `chsh`.

Fixes #12.

ansible/roles/dotfiles/tasks/zsh.yml

@@ -13,5 +13,9 @@
     dest: "{{ ansible_env.HOME}}/.{{ item }}"
     state: link
   with_lines: ls ~/.zprezto/runcoms | grep -v README.md
-- command: chsh -s zsh {{ ansible_env.USER }}
+- lineinfile: dest=/etc/shells line=/usr/local/bin/zsh state=present
   sudo: True
+  when: ansible_distribution == "MacOSX"
+- command: chsh -s /usr/local/bin/zsh {{ ansible_env.USER }}
+  sudo: True
+  when: ansible_distribution == "MacOSX"