tests: errors installing pylibmc lib

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:

```console
Warning, treated as error:
error while formatting signature for invenio_cache.proxies.current_cache: Handler 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.