global: fix build failure

[ntarocco]

Same as:

• invenio-records-ui #84
• invenio-records-rest #289
• invenio-search-ui #73
• invenio-previewer #111
• invenio-theme #145
• invenio-base #147
• invenio-i18n #81

https://travis-ci.org/inveniosoftware/invenio-search-ui/jobs/656679730

Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/_pytest/config.py", line 318, in _importconftest
    mod = conftestpath.pyimport()
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/py/_path/local.py", line 701, in pyimport
    __import__(modname)
  File "/home/travis/build/inveniosoftware/invenio-search-ui/tests/conftest.py", line 18, in <module>
    from flask_babelex import Babel
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/flask_babelex/__init__.py", line 23, in <module>
    from werkzeug import ImmutableDict
ImportError: cannot import name ImmutableDict
ERROR: could not load /home/travis/build/inveniosoftware/invenio-search-ui/tests/conftest.py
The command "./run-tests.sh" exited with 4.

Fix also deprecation warning in Travis config:
https://travis-ci.org/inveniosoftware/invenio-search-ui/jobs/656679730/config