Enable virtualenv and pip requirements at readthedocs

[datakurre]

@tisto, please, update ploneact documentation build settings at readthedocs with

Use virtualenv:

[ x ] Install your project inside a virtualenv using setup.py install

Requirements file:

docs/requirements.txt

These changes will allow the use of sphinxcontrib-robotdoc in our docs.

[datakurre]

Done.