global: fix import error

[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-theme/jobs/656629115

Warning, treated as error:
autodoc: failed to import module u'ext' from module u'invenio_theme'; the following exception was raised:
cannot import name ImmutableDict
The command "./run-tests.sh" exited with 2.

Fix also deprecation warning in Travis config:
https://travis-ci.org/inveniosoftware/invenio-theme/jobs/656629115/config