Only validates once when using a mocha watcher

[EyMaddis]

Hi,
great mixin, but I noticed that it does not handle reloads.
I use mocha -w when writing tests. I enabled the debug output for the mixin and have a test to verify it's functionality within our own test suite.
The first time always works, but if the tests re-run (due to file changes) it will not re-apply the mixin. There is also not debug output.
I think mocha uses the caching for npm modules.

Not sure whether this is a loopback issue (mixin loading) or can get solved here.

Notice: I use the model-config.json method for loading the mixin as stated in this issue #4