Replace version check routine by checking constant
Switch IP field to be nullable

Fixes #202
Affects only reactivation of the plugin, still need to do the same with auto-updates
since register_activation_hook doesn't work on update anymore since 3.1 [core#14915]
Enable upgrade procedure check on each run

Executes the install procedure, which switches to upgrade if found a previous db version
Finishes #202
Fix silly typo mistake
Merge pull request #203 from x-team/issue-202

Fix Stream fail on invalid IPs