Add a 'requirements:' field to homebrew doc (#2630)

homebrew.py and homebrew_cask.py make use of python
2.5 and 2.6 features like string .format() method.

packaging/os/homebrew.py

@@ -27,6 +27,8 @@ author:
     - "Indrajit Raychaudhuri (@indrajitr)"
     - "Daniel Jaouen (@danieljaouen)"
     - "Andrew Dunham (@andrew-d)"
+requirements:
+   - "python >= 2.6"
 short_description: Package manager for Homebrew
 description:
     - Manages Homebrew packages

packaging/os/homebrew_cask.py

@@ -24,6 +24,8 @@ author:
     - "Indrajit Raychaudhuri (@indrajitr)"
     - "Daniel Jaouen (@danieljaouen)"
     - "Enric Lluelles (@enriclluelles)"
+requirements:
+   - "python >= 2.6"
 short_description: Install/uninstall homebrew casks.
 description:
     - Manages Homebrew casks.