Fix- compatibility issue with webpack v3

[samundrak]

This PR will fix the error 'afterEmit' and will also make it compatible when used with webpack v3.
Fix - #3

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[developit]

Looks the the test failure is actually a fixed case. Just need to update the snapshots.

[developit]

@samundrak .catch(console.error).then(callback) will call callback, since catch swallows the error.

[samundrak]

Thanks @developit