Drop support for Mockito 1.x

[wilkinsona]

Our support for Mockito 1.x in Boot 2.0 is a bit muddled at the moment. We still have MockitoApi and Mockito1Api and Mockito2Api but this issue removed the compatibility tests for Mockito 1.10 (and earlier 2.x releases). We either need to completely drop Mockito 1.x or reinstate some of those compatibility tests.

[philwebb]

I vote drop 1.x support.

[snicoll]

Ditto