Release v0.15.0

[add] mojeek engine

[fix] google videos engine

Remove Ixquick engine

restore startpage search results

Fix bing image thumbnail path

- Forget one "\" in the process!

Instead of a single line with 500000 characters use nicely formatted JSON.
Sort the lists in engine_languages.py so when updating it is possible to
more easily see the differences (search engines do change the order their
languages are listed in)

[add] seznam engine

Update Dockerfile

- Fix for docker image build error "Could not find a version that satisfies the requirement cffi!=1.11.3,>=1.7 (from versions: )"

Minor changes

Add Gentoo wiki search engine

Searching for english is now giving all pages results.
Instead searching for a specific language different from english
is correctly returning only translated pages for the selected language.

Use new LABEL syntax for Dockerfile