Dependencies upgrade

https://pypi.python.org/pypi/pyOpenSSL : fix memory leak
http://lxml.de/4.1/changes-4.1.1.html : speed improvement

fix bug: TypeError: set(['bing', 'google']) is not JSON serializable,…

print() is a function in Python 3

Fixed the currency_convert engine.

Continuation of PR #1004 (seems abandoned)

results: make engines of results set

Previously, if a same result was returned from the
same engine, the name of the engine would be added
more than once. To eliminate engine name duplication
engine list was changed to set.

[add] OpenAIRE publication engine

and modifies OpenAIRE datasets engine

Finish PRs of @jibe-b: pubmed, oa_doi_rewrite, openaire, arxiv

Closes #1045

Closes #1047

This is the continuation of #1048 created by @jibe-b.

Closes #1048

Closes #1050

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[fix] Missing favicon images in oscar and simple themes

Same change as with the oscar theme.

The `filename` parameter of the `url_for` function doesn't need a leading `/`, or else the resulting URL features a double-slash `//` that throws off searx 0.12.0 with Apache 2.4.25 on Debian, resulting in missing favicons.

[fix] Language support for Bing Images and Videos