Update AppleDoc install instructions

@tomaz tagged a new version of AppleDoc which resolves the issue requiring
the `--HEAD` param to `brew`, and @adamv [pulled in the change](https://github.com/mxcl/homebrew/pull/14693)
to mxcl/homebrew that both updates to the new code and also uses
the [new compile-time template location](https://github.com/tomaz/appledoc/pull/231)
to avoid the second step.

This caused a Bug that lead to Pods with preferred dependencies to be always
installed.

Closes CocoaPods/Specs#464.

Should fix Travis errors.

closes #490.

This reduces ambiguity and prevents bugs like #489, which was caused by the
Sandbox creating a LocalPods from the spec in the root (if present) and
ignoring the one in Local Pods.

Closes #489.

The caching of LocalPods might need a refactor.

Closes #485

See #485

Closes #484

Fixes: https://secure.travis-ci.org/#!/CocoaPods/CocoaPods/jobs/2236387

* pod_update: (37 commits)
  [Specification] Provide a meaningful error message if a subspec can't be found.
  Update lockfiles of examples.
  Handle pods in (case-insensitive) alphabetical order.
  [outdated] Don’t show pods that can't be checked.
  Fix outdated command.
  Improve Podfile.lock version when the `:head` option is used.
  Improve error message when a local source dir doesn't exist.
  Refactor and cleanup command source files.
  Don't break when a command isn't recognized and trying to show a help banner.
  Changelog.
  [Specs] Minor fix for Lockfile.
  [LocalSource] Specs.
  Changelog.
  Changelog.
  [Lockfile] Fix specs for grammar change.
  [Lockfile] Grammar.
  [LocalSource] Tuning.
  [Lockfile] Encode checksums only if supported.
  [ExternalSources] Better robustness :-)
  [Lockfile] Save checksums as string.
  ...

Conflicts:
	CHANGELOG.md
	Gemfile.lock