virtualenv needs to be modified for pdftables

[StevenMaude]

https://bitbucket.org/sensiblecode/data-services-scraper-template/issues/35/virtualenv-needs-to-be-modified-for

ian_hopkinson:

If you try to run pdftables from a virtualenv then you get a fail on trying to import gtk. This can be fixed by using:

virtualenv --system-site-packages venv

I'm not sure this is ideal, hence I didn't add it myself. See this issue here where Francis and I discuss it:

https://github.com/scraperwiki/pdftables/issues/118