Fix coverage version

[jmbenlloch]

Travis has started to fail in IC installations due to a newer version of the coverage package, which is not listed in our dependencies list but it is need by one of them.

You can see the failures here:

https://travis-ci.org/jmbenlloch/IC-1/jobs/565919414?utm_medium=notification&utm_source=github_status
https://travis-ci.org/nextic/ANTEA/jobs/637520045?utm_medium=notification&utm_source=github_status

This PR simply fixes the coverage version manually so new installations won't get the failing one.

I'm copying one sample error here so we can have it archived in case we needed it in the future:

    @given(random_length_float_arrays(mask = lambda x: ((x<-10) or

>                                                  (x>+10) )))

    def test_in_range_with_hole(data):

invisible_cities/core/core_functions_test.py:60: 

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <hypothesis.core.StateForActualGivenExecution object at 0x7fe358f62910>

test_runner = <function default_new_style_executor at 0x7fe3811340e0>

search_strategy = tuples(just(()), fixed_dictionaries({'data': integers(min_value=0, max_value=100).flatmap(lambda n: float_arrays(n, **kwargs))}).map(lambda args: dict(args, **kwargs)))

test = <function test_in_range_with_hole at 0x7fe3596229e0>

settings = settings(buffer_size=8192, database=DirectoryBasedExampleDatabase('/home/travis/build/jmbenlloch/IC-1/.hypothesis/exam...INFER: 1>, stateful_step_count=50, suppress_health_check=(), timeout=60, use_coverage=True, verbosity=Verbosity.normal)

random = <random.Random object at 0x56103c016310>, had_seed = None

    def __init__(

        self, test_runner, search_strategy, test, settings, random, had_seed

    ):

        self.test_runner = test_runner

        self.search_strategy = search_strategy

        self.settings = settings

        self.last_exception = None

        self.falsifying_examples = ()

        self.__was_flaky = False

        self.random = random

        self.__warned_deadline = False

        self.__existing_collector = None

        self.__test_runtime = None

        self.__had_seed = had_seed

    

        self.test = test

    

        self.coverage_data = CoverageData()

        self.files_to_propagate = set()

        self.failed_normally = False

    

        self.used_examples_from_database = False

    

        if settings.use_coverage and not IN_COVERAGE_TESTS:  # pragma: no cover

            if Collector._collectors:

                parent = Collector._collectors[-1]

    

                # We include any files the collector has already decided to

                # trace whether or not on re-investigation we still think it

                # wants to trace them. The reason for this is that in some

                # cases coverage gets the wrong answer when we run it

                # ourselves due to reasons that are our fault but are hard to

                # fix (we lie about where certain functions come from).

                # This causes us to not record the actual test bodies as

                # covered. But if we intended to trace test bodies then the

                # file must already have been traced when getting to this point

                # and so will already be in the collector's data. Hence we can

                # use that information to get the correct answer here.

                # See issue 997 for more context.

                self.files_to_propagate = set(parent.data)

                self.hijack_collector(parent)

    

            self.collector = Collector(

                branch=True,

                timid=FORCE_PURE_TRACER,

                should_trace=self.should_trace,

                check_include=hypothesis_check_include,

                concurrency='thread',

>               warn=escalate_warning,

            )

E           TypeError: __init__() missing 2 required positional arguments: 'should_start_context' and 'file_mapper'

../../../miniconda/envs/IC-3.7-2018-11-14/lib/python3.7/site-packages/hypothesis/core.py:522: TypeError

[paolafer]

This PR fixes a problem with hypothesis, caused by newer version of the coverage package. The tests succeed, therefore I approve it.