Added pre-requisite package details for Ubuntu installation

I installed a few required packages to ensure that things work on Ubuntu.

However, I am stuck at the first Python step, after successfully running `pip install -r requirements.txt`:

    >>> from prflask import db
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ImportError: No module named prflask

Where does `prflask` come from?