Do not execute `berks update`.

Do not execute `berks update` command,
because it is redundant.

Just after `berks install`, `berks update` is executed.

This commit fixes #218.
Merge branch 'id/218' into dev
Download cookbooks under Berkchelf's management to cookbook directory.

By default, cookbooks under Berkchelf's management
is downloaded to ~/.berks.

By specifying the destination directory,
it can be customized.

This commit fixes #217.
Merge branch 'id/217' into dev