Upgrade Mockito to 2.0.99-beta

[sghill]

Recent versions of Mockito break with mockito-kotlin because of changes in mockito/mockito#404:

• org.mockito.internal.util.MockUtil is now a static utility class
• org.mockito.Mockito#stub has been removed in favor of #when

This change updates mockito-kotlin to not instantiate MockUtil and aliases com.nhaarman.mockito_kotlin.stub to com.nhaarman.mockito_kotlin.whenever, which was suggested by the docs.

I think it may be better to remove the stub method all together, since Mockito has chosen to do that, but figured this was a safer starting point. Happy to update the PR if that's the better route.

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[nhaarman]

Thanks!

Merely replacing the stub call by whenever will still break any client's code since their return types differ.

Since this library is essentially a wrapper for the original Mockito library, I think we should indeed remove the stub method and keep the api one-on-one.

[sghill]

Sounds good to me -- PR has been updated to remove stub