hoverEnabled not properly working

## I'm submitting a...

[X] Bug report

[ ] Feature request

There is a related issue: https://github.com/BioPhoton/angular-star-rating/issues/38

## Current behavior

Only in case 'hoverEnabled' is 'true':

* In Chromium: It works, but after hovering the stars, they change their color to default color (= grey).
* In Firefox: hover is not working at all

## Expected behavior

* In Chromium: The current color (positive = green, ok = yellow, negative = green) should be maintained after hovering.
* In Firefox: It should do something ;)

## Minimal reproduction of the problem with instructions

I've implemeted the star rating as follows:

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## Environment

Library version: 4.0.0-beta.3

Angular version: 5.2.0


Tested browsers:

- Chromium (desktop) version 65

- Firefox (desktop) version 59