Use Stefan Birkner's lib-parent as parent POM.

This parent POM provides up-to-date versions of plugins and configures
the plugins that are needed for releasing the library at Maven Central.
RestoreSystemProperties restores all properties.

Simplifies usage of the rule. Users just need to paste the snippet.

    @Rule
    public final TestRule restoreSystemProperties
            = new RestoreSystemProperties();

and every property is restored.
Deprecated clearProperty(String).

This method is no longer needed because RestoreSystemProperties
restores all properties. Now the user can use System.clearProperty
in its test.