CI breaks on python 2.7.10

[capelastegui]

On Travis CI, we are getting an ImportError due to a conflict in the version of the 'six' library, when running on python 2.7.10

ImportError: Failed to import test module: tests.test_forecast_plot
Traceback (most recent call last):
  File "/opt/python/2.7.10/lib/python2.7/unittest/loader.py", line 254, in _find_tests
    module = self._get_module_from_name(name)
  File "/opt/python/2.7.10/lib/python2.7/unittest/loader.py", line 232, in _get_module_from_name
    __import__(name)
  File "tests/test_forecast_plot.py", line 23, in <module>
    from anticipy.utils_test import PandasTest
  File "anticipy/__init__.py", line 2, in <module>
    __version__ = pkg_resources.require(__name__)[0].version
  File "/home/travis/virtualenv/python2.7.10/lib/python2.7/site-packages/pkg_resources/__init__.py", line 900, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/home/travis/virtualenv/python2.7.10/lib/python2.7/site-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
ContextualVersionConflict: (six 1.10.0 (/home/travis/virtualenv/python2.7.10/lib/python2.7/site-packages), Requirement.parse('six>=1.11.0'), set(['jsonschema']))

six is not a dependency of anticipy. It's probably used by some imported library. Since the code otherwise works fine on python 2.7.15 and 3, I think the best solution is to drop 2.7.10 from the CI script.