when executing "archive" for a package with multiple versions

, but without a specific version constraint, package the latest stable version by default, instead of the first version.

Example: `composer archive --format=zip seld/jsonlint`
Before change: packages v1.0.0, after change: packages v1.1.2.
![latest-vesion-as-default-for-archiving](https://cloud.githubusercontent.com/assets/85608/2870706/4260590e-d2d9-11e3-959a-db83c4d44831.jpg)