Use puppetlabs/git module for git package declaration

[sbadia]

This commit remove git package resource declaration inside this module. On Debian based system (Debian, Ubuntu), git-core is a meta-package on git. This commit also solve the duplicate declaration faced by Andrew in #114 (add rbenv support).

Compatibility is keept, but the git_package_name parameter is now deprecated and has no effect.

[igalic]

+1