PackageRequirement: Add get_installed_version
Adds get_installed_version in PackageRequirement and implement it in PipRequirement
Related to https://gitlab.com/coala/package_manager/issues/35
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Adds get_installed_version in PackageRequirement and implement it in PipRequirement
Related to https://gitlab.com/coala/package_manager/issues/35