Use puppetlabs/git module for git package declaration

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.
[travis] Fix output formatter (for RSpec 3.0.0)
Force travis/spec to use RSPEC version 2.14.x

Since our test are not rspec 3.0.0 ready, we need to force travis
to use rspec version 2.14.x
Merge pull request #157 from sbadia/git-core

 Use puppetlabs/git module for git package declaration