Tagging release versions to allow fetching a known version of Satis

To allow deploying a particular version of Satis it would be very useful if git tags could be utilised for each new release of Satis. This would for instance allow a CI process running a Satis build to be able to pull down a known version of Satis to run (using `composer create-project`).

This would be especially useful when breaking changes are made such as removing PHP 5.4 support as any CI servers with PHP 5.4 on them (CentOS/RHEL7) can still build from the last tag supporting 5.4 instead of always using master