Ensure git-shell is installed before setting the user shell

Recent Puppet versions validate shells, and the ordering's unspecified currently.

<pre>
[ERROR 2014-10-23 10:24:53 main]  /Stage[main]/Puppet::Server::Install/User[puppet]/shell: change from /sbin/nologin to /usr/bin/git-shell failed: Shell /usr/bin/git-shell must exist
[ WARN 2014-10-23 10:25:10 main]  /Stage[main]/Git::Install/Package[git]/ensure: created
</pre>