Bug 23230: Make _version_compare OO context aware

Bug 23230: Make _version_compare OO context aware

On using _version_compare in plugins development, it would be nice if it
was able to be called as an object method. This way we don't need to
fully qualify it like this:

    Koha::Plugins::Base::_version_compare

and we can do

    $self->_version_compare

instead.

This patch implements this behavior change. It is backwards compatible
so plugins using the 'old way' don't break.

To test:
- Apply the unit tests patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Plugins.t
=> FAIL: Tests fail
- Apply this patch
- Run:
 k$ prove t/db_dependent/Plugins.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>