Add a pre-install hook

Make updating specs repos its own UI section

[Cache] Remove cache when CocoaPods version changes

Symlink vendored framework headers into the public headers dir

Fixes #3161

[Travis] Only build master and stable branches

Flush stdin/stderr and wait a bit in `executable`.

This should make it unlikely that we don't catch the full output of
commands we are executing.

[Cache] Recognize incomplete cache for released pods

Even when cached_spec isn't used further with local podspecs, it still must be read from file to verify that it was correctly written before, because that also indicates that the cached pod is valid.

Shares schemes of any development pods.

The current user (`ENV['USER']`) is already the default.

This makes it easier to individually build the product of a development
pod from a checked-in project, e.g. when using the pod-template,
allowing for improved sharing with non-CocoaPods users.

[Doc] Improvements and Rake task to lint with inch