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-records-rest/jobs/656628971

Warning, treated as error:
/home/travis/build/inveniosoftware/invenio-records-rest/docs/api.rst:11:autodoc: failed to import module u'invenio_records_rest.ext'; the following exception was raised:
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 658, in import_object
    __import__(self.modname)
  File "/home/travis/build/inveniosoftware/invenio-records-rest/invenio_records_rest/__init__.py", line 700, in <module>
    from .ext import InvenioRecordsREST
  File "/home/travis/build/inveniosoftware/invenio-records-rest/invenio_records_rest/ext.py", line 15, in <module>
    from . import config
  File "/home/travis/build/inveniosoftware/invenio-records-rest/invenio_records_rest/config.py", line 14, in <module>
    from invenio_indexer.api import RecordIndexer
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/invenio_indexer/__init__.py", line 183, in <module>
    from .ext import InvenioIndexer
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/invenio_indexer/ext.py", line 19, in <module>
    from .cli import run  # noqa
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/invenio_indexer/cli.py", line 17, in <module>
    from invenio_pidstore.models import PersistentIdentifier, PIDStatus
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/invenio_pidstore/__init__.py", line 359, in <module>
    from .ext import InvenioPIDStore
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/invenio_pidstore/ext.py", line 18, in <module>
    from .models import PersistentIdentifier, logger
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/invenio_pidstore/models.py", line 18, in <module>
    from flask_babelex import gettext
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/flask_babelex/__init__.py", line 23, in <module>
    from werkzeug import ImmutableDict
ImportError: cannot import name ImmutableDict
The command "./run-tests.sh" exited with 1.

Fix also deprecation warning in Travis config:
https://travis-ci.org/inveniosoftware/invenio-records-rest/jobs/656628971/config