Deprecated getMock()  methods used in tests

See https://travis-ci.org/piwik/piwik/jobs/144259287 

eg 

```
22) Piwik\Plugins\Test\Columns\UpdaterTest::test_getMigrationQueries_ReturnsCorrectQueries_IfDimensionIsNotInTable
PHPUnit_Framework_TestCase::getMock() is deprecated, use PHPUnit_Framework_TestCase::createMock() or PHPUnit_Framework_TestCase::getMockBuilder() instead
```

we should probably update this soon
