(re)Added SUSERelease module

- Provides backward compatibility
- Should be only used during Upgrade
Added new testcase for SUSERelease
New function Update.installed_product

- Uses both SUSERelease and OSRelease
Added new testcase for Update.installed_product
Using Update.installed_product in other parts of yast2-update
Adapted .gitignore to test
Added Makefile for tests
Added rspec to build-requirements
And yast2-ruby-bindings are needed as well
Added also yast2
More specific yast2 version
More Yast-like module
Added SUSERelease.rb into Makefile
Added missing build-requires
New changes file entry and new version
Comments from review
Merge pull request #14 from kobliha/backport_suse_release

Fixed searching for systems available for upgrade (before os-release has replaced SuSE-release)