Upgrade Nightwatch to 2.1.x

[MMMalik]

Are you reporting a feature request or a bug?

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Provide detailed reproduction steps (if any)

Due to changes introduced in #286 a fixed verion of Nightwatch is used.
In order to use version 2.1.x (or higher) we should use different API. Consult docs for more details.

Also, one of the methods we rely on is deprecated:

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