tests: errors installing pylibmc lib

[mvidalgarcia]

https://github.com/inveniosoftware/invenio-cache/pull/17/checks?check_run_id=1524244138

python -m sphinx.cmd.build -qnNW docs docs/_build/html also generates some problems:

Warning, treated as error:
error while formatting signature for invenio_cache.proxies.current_cache: Handler <function record_typehints at 0x104db0a60> for event 'autodoc-process-signature' threw an exception (exception: Working outside of application context.

This typically means that you attempted to use functionality that needed
to interface with the current application object in some way. To solve
this, set up an application context with app.app_context().  See the
documentation for more information.)

Commenting out pylibmc installation and Sphinx build, all unit tests pass successfully.

[mvidalgarcia]

Also, it uses memcached which is not supported anymore by docker-services-cli.