Switch to use package mock of version 1.1.0 or greater

[Bulat Gaifullin]

since version 1.1.0 introduced fix, that prevent to call any method, that starts with 'assert' and not implement in mock class.
for example:
in version 1.0.1 this code runs properly:

some_mock.assert_this_method_is_not_defined()

it will create a new mock method and test pass even it checks nothing

in new version some code will generate AttributeError unless special flag usafe_mock is set.

[Alexander Kislitsky (akislitsky)]

Now we can write 'assert_called_with_once' in code and believe, that test performs check.
For instance: [1]

[1] https://github.com/stackforge/fuel-web/blob/f882c428db97ee1eb93a4871f9d5857c5a7771b2/nailgun/nailgun/test/unit/fuel_statistics_tests/test_client_provider.py#L91

Set priority to Medium.