Properly check if the slider values have changed

[jespirit]

Fixes #878

I'll add unit tests later.

You can check the JSFiddle in the issue.

Pull Requests

Please accompany all pull requests with the following (where appropriate):

• [ ] unit tests (we use Jasmine 2.x.x)
• [x] JSFiddle (or an equivalent such as CodePen, Plunker, etc) or screenshot/GIF with new feature or bug-fix
• [x] Link to original Github issue (if this is a bug-fix)
• [ ] Passes CI-server checks (runs automated tests, JS source-code linting, etc..). To run these on your local machine, type grunt test in your Terminal within the bootstrap-slider repository directory

[seiyria]

Good catch!