Update tabs.js

[zsong]

Fixed the issue of "Cannot read property '$parent' of undefined" error when use the tabset directive inside the accordion directive. Tested with AngularJS 1.2.2.

[vinteo]

this also happens when you have a tabset within a tab of a parent tabset

[Coridyn]

Thanks, your change sorts it out for me.
I've encountered this issue when the tabset is inside ng-if:

<div ng-cloak ng-if="isReady">
    <div class="main-panel">
        <div tabset type="'pills'">
            <div tab>
                <div tab-heading>Heading 1</div>
                <div>...content...</div>
            </div>

            <div tab>
                <div tab-heading>Heading 2</div>
                <div>...content...</div>
            </div>
        </div>
    </div>
</div>

[chrisirhc]

Note to self to check again after #1362 gets merged (if it does get merged).

[chrisirhc]

@zsong , could you check if this is still an issue on master since #1362 has been merged?

[pkozlowski-opensource]

@zsong @chrisirhc I think this definitively can be closed now. @zsong feel free to open another issue if you still face problems with the latest code from master.