global: fix version conflict

[ntarocco]

https://travis-ci.org/inveniosoftware/jsonresolver/jobs/656682629

File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/setuptools/dist.py", line 833, in get_command_class
    ep.require(installer=self.fetch_build_egg)
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2465, in require
    items = working_set.resolve(reqs, env, installer, extras=self.extras)
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (six 1.8.0 (/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages), Requirement.parse('six>=1.9.0'), set(['bleach']))
The command "./run-tests.sh" exited with 2.

Fix Travis deprecation:
https://travis-ci.org/inveniosoftware/jsonresolver/jobs/656682629/config