Make plugin dependency fail with new Mantis release

This is a follow-up of discussion in [Issue 17360](http://www.mantisbt.org/bugs/view.php?id=17360) (see also #234), which implements @grangeway's [suggestion](http://www.mantisbt.org/bugs/view.php?id=17360#c40643).

If a plugin's minimum dependency for *MantisCore* is unspecified or lower than the current release (i.e. the plugin does not specifically list the current core version as supported) and the plugin does not define a maximum dependency, we add one with the current version's minor release (i.e. for 1.3.1 we would add '<1.3').

The purpose of this is to avoid compatibility issues by disabling plugins which have not been updated for a new Mantis release; authors have to revise their code (if necessary), and release a new version of the plugin with updated dependencies.

(sorry) '>='